Spy checking

A BLU Pyro successfully Spy checks teammates leaving their spawn area on Harvest.

“

Hard ta stab a guy in da back when he's beatin' ya freakin' head in, huh?

Click to listen

— The Scout on anti-espionage.

”

Spy checking is the common term for a thorough inquisition made by teammates in order to determine the whereabouts of enemy Spies and eliminate them. It is important to remember that Spies cannot attack while cloaked, cannot immediately attack or place Sappers after decloaking, and can effectively kill by attacking from behind. The process is somewhat complicated since disguised Spies can use the Dead Ringer to make a hasty escape when discovered.

Identifying Spies

General Strategy

Remember that players on opposite teams cannot pass through each other. Bumping into teammates indicates that that player is an enemy Spy.

Look for teammates that are behaving bizarrely. Players in unusual positions, moving in the wrong direction, deliberately avoiding contact with teammates, or hiding in corners should all be Spy checked.

Watch for teammates who are not firing their weapons or going toward resupply areas when hurt. Spies can not fire their weapon without removing their disguise, and they can not enter an enemy respawn area even when disguised.

Pay attention to the text chat. Spies will sometimes use voice commands, which will appear to all members of the opposing team as originating from the player that the Spy is disguised as. If you see voice commands originating from your name, alert teammates that the Spy is likely disguised as yourself.

Keep an eye out for telltale shrouds of smoke that are colored red or blue. When a Spy uses the Disguise Kit, it takes a second for the disguise to equip. At that time, the Spy will be shrouded with his team-colored smoke. While the smoke does not appear if the Spy is already cloaked, it will follow the Spy for a short distance if he uses it before cloaking.

If you have been killed, you can cycle through different camera angles around the map while waiting to respawn. Look out behind teammates or on open points for Spies uncloaking or disguising. Then you could Spy-Check the last known location of the Spy and the location around or near it.

Class-specific strategies

Scout

The Scout is the fastest class. Because of this, one effective strategy is to run around the map and try to bump into fellow teammates. As noted above, any teammate that cannot be passed through is an enemy Spy in disguise.

Equip the Sandman and attempt to stun any teammates suspected to be a Spy. The baseball will stun disguised Spies, but will simply pass through teammates.

Throw Mad Milk at at a teammate to Spy check them. Mad Milk will expose a disguised Spy, similar to Jarate.

Two close-range hits from the Scattergun, the Force-A-Nature, or three from the Shortstop will kill a Spy not equipped with the Dead Ringer.

Dead Ringer-equipped Spies can be identified easily if the player equips a Candy Cane, Holy Mackerel, or Unarmed Combat; a health pack will not drop upon kill with the Candy Cane, while the FISH KILL! and ARM KILL! notices will not appear upon kill with the Holy Mackerel or Unarmed Combat, respectively.

A teammate who gets hit by the bauble of your Wrap Assassin and subsequently bleeds is a Spy.

Soldier

Fire rockets into teammates and see whether or not the rocket explodes on contact with them, since rockets will simply pass through teammates. However, keep in mind that the rocket launchers have low ammo counts, so Spy checking in this manner can burn through ammo rather quickly.

The Buff Banner, Battalion's Backup and Concheror will not affect enemy Spies, so pay attention to whether or not nearby teammates begin glowing with the appropriate particle effect by the activation of either of these three weapons.

Hitting an enemy Spy with the Disciplinary Action will bestow neither you nor the Spy with a speed boost.

While killing a Dead Ringer Spy with the Half-Zatoichi bloodies the blade and allows you to sheath the weapon, the player will not attain the health boost the katana should grant. Keep an eye out and spray the area with rockets or Shotgun shells.

Pyro

The Pyro's ability to set enemies on fire with incendiary weapons is perhaps the most useful tool for detecting Spies, since only enemies can be ignited. This is the most reliable method of Spy checking teammates. The team-colored glow produced by a burning player is especially visible when surrounding a cloaked or disguised Spy. It is recommended to use only small bursts to Spy check, as a Flamethrower requires great amounts of ammunition to fire at a constant rate. Though keep in mind this tactic will not be as effective on Spies equipped with The Spy-cicle. If you Spy check a teammate and they attempt to run away afterwards, follow them and attempt to light them again, as The Spy-cicle has a 15 second recharge time.

The Compression blast is also useful in revealing Spies, as well as preventing them from accomplishing objectives. Teammates will not be affected by compression blast, so any teammate that can be juggled is a Spy. It is recommended to simply ignite the Spy, as compression blast costs a considerable amount of ammo; however, this can be used to push them away from your teammates if they are in danger of being backstabbed.

If a Spy is using the Dead Ringer, they may simply die instantly from a burn. Spray fire in the vicinity to try and reignite a Spy who has feigned death. Pursue them until you are certain of their death.

The Powerjack will not give the user a health boost upon killing a Dead Ringer Spy; spray the area with flames to negate this.

Demoman

In close quarters, the melee weapon is probably the most effective method. In particular, the Ullapool Caber, which does not detonate on friendly players, will detonate on Spies. However, remember that the Ullapool Caber will inflict 100 self-damage upon exploding.

In narrow corridors, a set of grenades from the Grenade Launcher can serve to force a Spy to clear the area or risk being discovered by blast damage. This tactic, if the Spy's Cloak is low, can force the Spy to become visible.

Placing Stickybombs with the Stickybomb Launcher on a wide area can cause Spies to hesitate about traversing such a minefield. Teammates will simply travel over the sticky trap.

A Demoman using the Chargin' Targe or Splendid Screen will be stopped mid-charge when bumping into a Spy.

While killing a Spy with the Dead Ringer with the Half-Zatoichi bloodies the blade and allows you to sheath the weapon, the player will not attain the health boost the katana should grant. Keep an eye out and spray the area with grenades.

A Cloak and Dagger Spy can be negated effectively by planting Stickies in corners that you are certain Spies often use, forcing them to move to different positions.

Heavy

Heavies can attempt to Spy check by firing their Minigun on suspicious teammates - a few seconds at close range will make quick work of an enemy Spy. However, make sure to have ammo remaining afterward.

If a teammate eats a dropped Sandvich and was already at full health, that player is a Spy.

Because movement speed is greatly reduced, a spun-up Heavy is a tempting backstab target. This can sometimes be used as a goad for enemy Spies to approach.

As Heavies are often coupled with Medics, Spies place great priority upon stabbing the Medic first, then the Heavy as to deny the team the Medic's support. Watch your Medic's back.

If a Spy dies due to one bullet of your Minigun, spray the area rapidly lest he be using a Dead Ringer. The bullets fan out as well as the Pyro's flames, allowing you to discern the Spy's location.

A kill on a Dead Ringer Spy will not activate the Killing Gloves of Boxing's critical effect.

A teammate that laughs after being punched by a Heavy with the Holiday Punch equipped is an enemy Spy.

Engineer

As an Engineer, Spies are a substantial threat, as they are able to place Electro Sappers while disguised. However, this can usually be mitigated if teammates can be asked to constantly patrol the vicinity in order to discourage Spies; this can be especially helpful if Pyros can be recruited for this task.

Keep an ear out for Sapper notifications; if one happens while a teammate is standing near the sapped building, that teammate is probably a Spy.

Spy check anybody that approaches your buildings with the Pistol, Shotgun, or Wrench, especially teammates that are approaching friendly buildings under suspicious circumstances.

If any teammate is unable to pass through a Sentry Gun or Dispenser, that teammate is a Spy; the only exceptions are Engineers and their own buildings.

Placing Sentry Guns onto an end of a long 'hall' with the Wrangler is convenient, due to the minor knock-back of the bullets and the Rockets.

It is also a good idea to shoot around the immediate area with the Pistol, Shotgun or Wrangler (if equipped) to check for Spies using the Cloak and Dagger.

A teammate that starts bleeding upon being hit by the Southern Hospitality is an enemy Spy. The bleeding effect will persist when the Spy cloaks.

The Widowmaker is a superior weapon when Spy checking as you will regain your metal if you hit the Spy, whereas if you shoot your teamates you will not, respectively.

Medic

As a Medic, Spies can use you to gain health while disguised. Both your primary weapon and melee weapon can Spy check easily.

Spies may target you while you are healing a patient. Watch your back while healing. You can also rely on your patient to Spy check; however, this could prove difficult during battle.

Spies may also simply head directly for your patient rather than yourself. Protect your patient, stop healing and quickly take out the Spy. You can also use the "Spy" voice command to alert them before they get close enough. If they do take out your Medic buddy, try to pedal backwards while firing Syringes to avoid being stabbed yourself.

Your Eternal Reward- or Wanga Prick-equipped Spies can stab your Medic buddy and will not interrupt your healing. If your Medic buddy suddenly acts suspicious, or if your teammate alerts you to the fact, take out your weapon and kill him.

When using the Crusader's Crossbow, it may be hard to Spy check. Spies will take damage from your arrows, instead of health. If a teammate is not being healed by your arrows (unless you are right next to your target), the player may be a Spy.

The Übersaw can be used on exposed Spies to gain some ÜberCharge. However, the longer swing speed may mean your life if the Spy is agile enough to circle around you.

When using the Amputator, your taunt heals teammates in close range. Any Spies disguised will not glow the usual team-colour during this taunt. Be wary, though, since you are vulnerable while taunting.

Try to heal friendly Soldiers holding the Equalizer if you can then they are a Spy.

Sniper

As a Sniper, your lack of mobility means you are an easy Spy target, especially while scoped-in. Use your SMG to shoot suspicious teammates around you, or douse them in Jarate.

Many Snipers only look ahead of them to discern any possible targets, and thus do not notice Spies beside them. Make sure to check your surroundings as you switch positions.

The Razorback may seem like the perfect anti-spy tool, but it is only really effective against Spies using the Your Eternal Reward (as they are unable to backstab you for a disguise), or the inexperienced and stupid. A competent Spy will just shoot you dead in 3 shots (or 2 if they are using the Ambassador or Enforcer), before you have time to react. Use other spychecking methods and be aware of your surroundings instead.

While using the Huntsman, an arrow will stick into a Spy, instead of going through them like it would toward a teammate. Use this to Spy check any teammates that are near you.

While the Kukri works well on killing Spies, the Tribalman's Shiv is more effective as the Spy will bleed, even while cloaked.

The Bushwacka is a good follow-up to a dousing of Jarate; the quick kill will make sure you won't be too distracted with hunting the Spy down.

If using the Machina, bring a weapon that can Spycheck well, such as the Tribalman's Shiv or SMG, as you cannot fire unscoped with the weapon.

Spy

The Spy can do a considerably good job of Spy checking. For example, your Knife, or its variants, can be used as an excellent screening tool; if you believe an ally is truly an enemy Spy in disguise, simply approach him/her from behind while your Knife is active. If you raise your Knife into the backstabbing position, you have effectively "seen through" the enemy's disguise. Spy checking someone who is actually a teammate will not yield this result.

If you suspect there is a Spy nearby, make use of the Dead Ringer. While active, the Dead Ringer can protect you from a backstab, while at the same time fooling the enemy into believing he has killed you. At this point, you can either rely on your team to kill the enemy Spy, reveal his location, or dispatch him yourself.

When facing a Spy using the Dead Ringer, remember that upon backstab, the Conniver's Kunai will not leech health, Your Eternal Reward and the Wanga Prick will neither disguise you nor dissolve the enemy's body, and the Big Earner will not grant Cloak.

If moving into an area not dominated by one team with the Cloak and Dagger, make sure that the corners do not contain other enemy cloaked Spies that might give away your position.

Revealing cloaked Spies

When cloaked, Spies will "flicker" their team color upon being damaged or when bumping into members of the opposite team. Keep an eye out for such flickering figures who appear in the opposite team's color. Try to strafe while navigating corridors in case a Spy is trying to go through that same corridor.
- The Dead Ringer does not have this drawback; however, it only lasts for the first 6.5 seconds of the Cloak, after which Spies will flicker when damaged or bumped into by members of the opposite team.
- Conversely, remember that friendly Spies will always be partially visible to teammates while cloaked.

Try to listen for the sounds of decloaking. Spies' cloaks will emit unique sounds when cloaking and decloaking, and such sounds indicate the presence of a Spy.

An Ammo box, Health kit, or a dropped weapon that disappears without a player walking over it indicates that it was picked up by a cloaked Spy. (Exception: if it was a dropped weapon, it may have simply disappeared because it was not picked up in time.) Spy check the immediate area to try to reveal the Spy.

In places with bodies of water, look for floating water drop particle effects. Players exiting bodies of water will be dripping with water, so any cloaked Spies that have recently exited a body of water can be easily identified by this.

An automatic door that suddenly opens without any visible player being nearby indicates a cloaked Spy passing through.

In general, weapons with a wide spread such as a Shotgun or Scattergun should be used to occasionally fire into corners where Spies using the Cloak and Dagger may be hiding.

As a Scout, try to throw Mad Milk into corners and crevices. It will immediately reveal any cloaked Spies and render their Cloak useless. The Scout's speed also allows players to patrol narrow corridors and bump into Spies attempting to make their way through side paths.

As a Pyro, try to randomly puff flames while patrolling, as there is always a chance that the flames will ignite a Spy trying to pass through. Puffing into corners and crevices will also usually uncover a Spy that is using the Cloak and Dagger and is waiting to recharge it. Beware that the Dead Ringer can put out flames if they are ignited, so if an ignited Spy suddenly drops dead, spray down the area with flames to ignite him again and use the compression blast to keep him at bay until the charge on his Dead Ringer runs out.

As a Heavy, spraying an area with the Minigun where a cloaked Spy may be can prove effective in exposing (and killing) him. The Minigun has a wide spread that can reveal Spies at long range.

As an Engineer, Spies can camp by Dispensers while cloaked for an unlimited time using the Invisibility Watch or Dead Ringer, or anywhere with the Cloak and Dagger. Try and check above and around friendly buildings to counter this.
- The Wrangler is also useful for spraying down areas, similar to how the Heavy can Spy check with the Minigun.

As a Sniper, similar to Scouts using Mad Milk, Jarate can short out an enemy Spy's Cloak, making him easy prey. Always try to throw Jarate into corners and crevices where Spies might be hiding before beginning sniping.

As with the Southern Hospitality, the Tribalman's Shiv can render a Spy's Cloak useless. Try to hit a Spy with at least one melee hit to reduce his chances of successfully escaping. However, if he has the Dead Ringer equipped, it will remove the Shiv's bleeding effect.

Protecting the Engineer's buildings

In general, simply try to Spy check anybody approaching a friendly Engineer's buildings. Spies trying to sap a building will usually make a bee-line towards the building they want to sap, so if possible, try to body-block the Spy to prevent him from placing a Sapper.

As a Pyro, use Dispensers to constantly refill the ammo of the primary weapon. This can be used to either create a standing firewall that Spies cannot pass through, or as periodic stops while constantly Spy checking the area.
- The Homewrecker or Maul can knock off enemy Electro Sappers with a single hit.

As a Demoman, the Stickybomb Launcher or Scottish Resistance is extremely effective at deterring enemy Spies. Simply place a few bombs around an Engineer's buildings and detonate them when the Engineer announces that his buildings are being sapped. While this may not be enough to prevent the buildings' destruction, it will at least prevent the Spy from escaping and causing more harm. It may also force the Spy to go completely around the buildings in fear of detonation by a watchful Demoman.

Similar to the Pyro, Heavies can use Dispensers to continuously refill their primary weapon's ammo, allowing players to Spy check over greater distances.

As an Engineer, always try to kill the Spy first before destroying any Electro Sappers placed on friendly buildings, as Spies will usually try to re-sap buildings immediately, or even try to kill the Engineer that is removing the Electro Sappers.
- If a Teleporter is being sapped from the Entrance, be sure not to stand on top of the Exit while destroying the Sapper; some Spies will deliberately try to telefrag players on the other side. Instead, stand next to the Teleporter Exit and melee the first person that teleports through, as it will likely be the Spy. If you want to repair your teleporter, stand near it and crouch. Furthermore, if the Spy does teleport through but dies on the first hit, he is likely using the Dead Ringer, so try to box him in and continue to attack to prevent him from escaping.

Identifying enemy Spies

Signs that definitely indicate an enemy Spy

Common signs

Various team-colored indicators will immediately give away an enemy Spy:
- The flicker of a Cloak from being bumped or injured.
- The fading of a Cloak while cloaking or uncloaking.
- The glow from being set on fire.
- The smoke from a recent use of the Disguise Kit.
- The cross from picking up a health kit.
A 'teammate' that cannot pass through another teammate, or conversely, a 'teammate' that has been observed walking through an enemy.
A 'teammate' whose weapons are not glowing and crackling with electricity when a team-wide crit boost occurs, e.g. after an Intelligence capture by your team. Keep in mind that some weapons are bugged and will not glow when crit boosted (like the Wrap Assassin), though they will still crackle.
A 'teammate' affected by another teammate's compression blast.
A 'teammate' that detonates friendly rockets and friendly grenades that have not bounced.
Seeing the cloaked silhouette of a non-Spy, or seeing a non-Spy cloaking or uncloaking. Friendly Spies always appear to be Spies, no matter their current disguise; any other cloaked class must be a disguised enemy Spy.
A 'teammate' who cannot enter a friendly resupply room, or who can enter the enemy's.
A 'teammate' placing Sappers on friendly buildings.
A 'teammate' standing on a friendly Sentry Gun or Dispenser, or who cannot pass through them, unless it is the Engineer who owns the building. No other team member can collide with Sentry Guns or Dispensers.
Two people that have the same name. One (or both) of them is a Spy. (Players that join a server that has their name already in use by someone have a bracketed number appended to their name.)
Seeing or hearing a voice command attributed to you without having used it.
Hearing a teammate's voice command when there are no players visible nearby.
A non-Spy 'teammate' who picks up a health kit while at full health (unless it is a friendly Heavy replenishing his Sandvich).
A teammate that is known to be dead or elsewhere, e.g. just respawned.
A teleporter that flashes but no one comes out, indicating an invisible enemy Spy came through.
A 'teammate' who has a name that does not correspond with his or her current class on the scoreboard. This happens when a Spy disguises as a class not currently on the team, or if the person has changed class while the Spy was disguised.
A 'teammate' on an uncontested control point or near the cart on Payload maps who does not seem to be capping the point or pushing the cart (unless it is a RED player on an Attack/Defend map).
A health or ammo pickup that vanishes without cause; this means a cloaked Spy has grabbed it.
A 'teammate' that is somewhere where normal teammates couldn't be (e.g. near the enemy spawn at the start of the round)
A spurt of blood from nowhere accompanied by a cracking noise; this is probably a careless Spy that has just taken fall damage.
A 'teammate' holding a weapon they cannot have, such as a Sniper with the Razorback holding the SMG. This can occur when the Spy tries to switch to a weapon slot containing a passive item.
A microphone indicator that is not connected to a body on an alltalk server (cloaked Spies still display the team-colored '!' indicator).
A microphone indicator belonging to an opponent that is connected to a teammate on an alltalk server.
A 'teammate' that is using their microphone but has no corresponding microphone indicator.
A 'teammate' whose weapon sounds like the Revolver when reloading, making a single clicking sound.
A 'teammate' who is overhealed when there are no friendly Medics on the team (though there are various weapon-specific exceptions). This is caused by a bug which can assign (for example) the health of a Heavy to a Scout disguise, if there are no Scouts on the target team.
A non-Spy 'teammate' that is being ignored by an enemy Sentry Gun.
A non-Spy 'teammate' being healed by an enemy Medic or Dispenser.
A player on the humiliated team still holding their weapon.

Class- or item-based signs

A burning Pyro. Real Pyros cannot be ignited.
A friendly Engineer with the Hotrod flipped over his face, but is not holding a toolbox; this is an enemy Spy holding a Sapper.
A 'teammate' with the wrong movement speed:
- A Scout or Medic running slower than usual.
- A low-health Soldier with the Escape Plan who isn't getting a speed boost.
- A Pyro using the Gas Jockey's Gear running at normal speed.
- A Demoman with the Scotsman's Skullcutter running at normal speed.
- A Heavy with the Gloves of Running Urgently running at normal speed.
A Soldier holding the Equalizer or Escape Plan and being healed by a Medic.
A 'teammate' holding a weapon that cannot have yet recharged. Examples include Mad Milk, the Sandvich and Buffalo Steak Sandvich, and Jarate - if they have been used recently, seeing that player holding it again too soon indicates it is a Spy.
A 'teammate' that can be stunned with a friendly Sandman's baseball or bleeds from a friendly Wrap Assassin's bauble.
A stun graphic with no attached body; this is sure to be an unlucky invisible Spy struck by a friendly Sandman's baseball.
A 'teammate' who is not healed by bolts from the Crusader's Crossbow; when real teammates are hit by a crossbow bolt, there is no impact sound.
A 'teammate' being affected by an enemy Medic's Amputator healing effect or, conversely, not being affected by a friendly Medic's Amputator.
The Medic's Amputator taunt buff appearing without a player, representing a cloaked enemy Spy.
A 'teammate' being soaked by friendly Jarate or Mad Milk, or not being affected by the enemy's.
A 'teammate' who appears unaffected by a friendly Disciplinary Action, and does not grant the Soldier a speed boost.
A 'teammate' who can switch from the Half-Zatoichi to another weapon without having made a kill.
A 'teammate' that laughs when punched with the Holiday Punch.
A piercing kill without an associated regular kill from the Machina, indicating a cloaked Spy was in the middle.
An enemy Scout or Pain Train user that is capping a point at x1 speed.

Signs that possibly indicate an enemy Spy

None of these indicators are conclusive proof of a Spy, but all of them would make a good time to run a Spy check:

Common signs

A teammate trying to stay behind other teammates, as if trying to backstab them.
A teammate not firing at the enemy.
A teammate who is in the direct firing line of an enemy but is not being fired at.
A teammate who retreats backward upon seeing other teammates, while still facing those teammates.
A teammate who appears to be dodging your team's attacks or backpedals immediately after seeing loose grenades or a carpet of friendly Stickybombs.
A teammate that deliberately avoids walking through other teammates, especially in tight corridors. Many Spies will strafe and dodge other players to avoid bumping into them.
A teammate that appears from a strange location like behind a rock, or in shadows.
A teammate hanging around areas that are advantageous to Spies, e.g. nearby dark unnoticeable corners.
A teammate that can be seen taking a route but not exiting it or a different player exits it. This could be a Spy cloaking or changing disguises.
An apparently injured teammate running around the spawn area doing nothing or calling for a Medic, instead of getting health from the resupply.
A teammate that isn't displaying the player's usual mannerisms. Maybe a player always advances with their secondary weapon out, or spams the "Move up!" voice command, or jumps constantly - if they are seen doing otherwise, they may be a Spy.
Someone standing still at a distance from the rest of the team. This could be a Spy trying to line up an Ambassador headshot.
A teammate that appears to be looking more at your team than at the battle.
A teammate calling for a Medic while at full health, who is not a Soldier, Demoman, or Heavy looking for overheal.
Anyone hovering near Sentry Guns or Dispensers, especially if they are attempting to move behind the Engineer tending them, or are at full health and do not seem to be collecting ammo.
Anyone running in an unexpected direction, such as a friendly Pyro, with full or nearly full health, running away from enemies.
Anyone who comes from an unexpected place, such as a Sniper coming down from the grate in 2Fort.
Someone acting inappropriately, such as a Pyro who stays near Snipers without lighting their arrows, a Demoman who is near a control point and has not placed Stickybombs, a Sniper at the front lines when the team is not advancing through a clear area, a Sniper near a control point or the Intelligence,, etc.
A teammate that is going towards the friendly Intelligence room without a reason (usually only Spy-checking Pyros, Demomen with Stickybomb Launchers, Engineers, and players who are carrying the enemy Intelligence have a reason to go to the Intelligence room).
A teammate who takes unusual routes without reason, e.g. going through the gap behind the house on Dustbowl Stage 2 after the first point has been captured may suggest a Spy.
A teammate that doesn't switch weapons much or is holding a weapon that is not suitable for the situation, e.g. a Soldier with the Shovel out while pushing the Payload.
A series of dropped weapons or building gibs disappearing in staggered intervals. While they could be disappearing naturally, it's also possible a Spy is picking them up.
A player being attacked by real teammates who believe the player is a Spy.
A teammate who has not been observed shooting but is collecting ammo could be a Spy refilling his Cloak.
A teammate who someone else accuses of being a Spy.

Class- or item-based signs

A Scout that doesn't double-jump.
A teammate deliberately trying to avoid friendly Pyros.
A Pyro that doesn't Spy-check often.
A Medic who isn't healing anyone.
An Engineer or Homewrecker/Maul Pyro that doesn't appear to be attacking Sappers.
A Heavy that does not spin his primary weapon when enemies appear, or is not spinning his gun when standing next to a Dispenser with the team expecting an attack.
A low-health Heavy with the Sandvich out or equipped and is not eating it.
A Sniper who is not zoomed in.
Someone who hesitates before running past friendly Snipers with the Razorback.
An Engineer that does not appear to be upgrading or repairing his buildings (or helping others upgrade or repair theirs) when it is a good time to do so.
An Engineer holding the Wrangler without a Sentry Gun following his aim. Consider that the Engineer could just not have a Sentry built, or have a Sentry elsewhere.
A teammate holding a weapon you know they are not using, such as a Sniper holding the default Sniper Rifle when you know the player always uses the Huntsman.
A Spy near an enemy building but isn't sapping it.
A Spy near an important enemy (e.g. Medic with full Übercharge) but hasn't backstabbed him.

Signs that indicate a teammate is not a Spy

These actions prove that a teammate is definitely not a Spy, and may also be performed to inform teammates that you are not an enemy Spy.

Common signs

Any usage of a weapon or item (shooting, swinging, eating, drinking, etc).
Can pass through teammates, as opposed to being stopped. (Beware that both teammates may be, in fact, Spies.)
On fire and glowing the friendly team's color.
A player entering a friendly resupply/respawn area.
Taunting (Disguised Spies cannot taunt).
If being fired at by enemies and is taking damage, e.g. bleeding.
Being able to pass through friendly buildings save for the Engineer who built them.
Carrying the enemy team's Intelligence.
A friendly player who is using their microphone, whose voice icon is the same color as the friendly team: A symbol appears over the player's head. In servers with sv_alltalk enabled, the symbol is tinted red or blue according to the player's true team allegiance.
A player saying certain responses, such as an Engineer saying his buildings are being sapped.
A teammate being boosted by a friendly banner.
Any teammate dying and leaving a ragdoll of the friendly team's color.
A teammate that can be spectated.

Class- or item-based signs

Isn't soaked by friendly Jarate or Mad Milk.
Any usage of class-specific abilities, e.g. Scouts double jumping, Heavies spinning their Minigun, Snipers that are scoped in, etc.
A Scout or Medic moving at his normal movement speed.
- A low-health Soldier with the Escape Plan, a Gas Jockey's Gear Pyro, or a G.R.U. Heavy moving faster, and a Demoman with the Scotsman's Skullcutter moving at a reduced speed.
A Scout under the effects of Bonk! Atomic Punch or Crit-a-Cola.
A Scout or Pain Train user capping a point at the expected value.
An Engineer holding a PDA or hauling a building.
An Engineer who automatically calls out when his buildings are sapped or destroyed, even when the buildings are out of his line of sight.
A Demoman's eye glowing from decapitating enemies with the Eyelander or any of its variants.
A Soldier with an active banner flag on his back.
Spies with their Disguise Kit out.
Spies without a mask on (not disguised).
An Engineer Wrangling a Sentry Gun, provided the Sentry Gun's laser sight is visibly following the Engineer's aim.
A friendly player who does not get stunned by taunts or the Sandman.
A teammate who can be healed by the Crusader's Crossbow.
A friendly Soldier and a teammate receiving a speed boost when the former strikes the latter with the Disciplinary Action.

Specific anti-Spy strategies

Main article: Anti-Spy strategy

Dealing with the Dead Ringer

The Spy has the ability to feign death with one of his unlockable items, the Dead Ringer, so always be wary upon killing enemy Spies as they may still be alive elsewhere. Thus, it's best to know some of the signs that tell that the Spy may not really be dead. Spies using the Dead Ringer cannot manually Cloak, and cannot use any weapons while the Dead Ringer is out.

If an enemy dies upon the first hit while approaching friendly lines without firing his weapon, it is most probably an enemy Spy disguised as one of his teammates. Most players don't run in with such low health, and a Spy disguised as a teammate will drop a corpse of the class he was disguised as.
- Similarly, if an obvious enemy Spy disguised as a teammate approaches the front lines from the enemy's position, he is most likely using the Dead Ringer, even if he doesn't die from the first hit. Remember that the Dead Ringer cannot Cloak at will, so a Dead Ringer Spy will rely on opponents' fire activating his feign death in order to become invisible.
- In general, if a player that has been identified to be an enemy Spy seems to be approaching a teammate suicidally, he is probably trying to get damaged so that his Dead Ringer will be activated, so don't be fooled.

A Spy must take the time to pull out his Dead Ringer before it becomes active, and cannot attack while he has it out. If a Spy suddenly stops attacking before dying on the next hit, he is probably using the Dead Ringer.

The decloaking sound of the Dead Ringer is highly distinctive, but requires players to be within a certain radius to hear it. Try to patrol near rarely-traversed areas to ambush a Spy decloaking in an area that he thinks is safe.

Picking up the Spy's dropped weapon does not replenish ammo or metal. This is an easy way to double-check if a recently-killed Spy is using the Dead Ringer or not.

Spies with the Dead Ringer will not flicker when damaged or walking into enemies while cloaked for the first 6.5 seconds of the Cloak, however, they still cannot be clipped through. Being stopped while moving and not bumping into any visible objects means that a Spy whose Dead Ringer was activated is trying to pass through.

If the Spy uses his Disguise Kit shortly before being "killed," the smoke effects will follow the Spy for the time that it takes for the Disguise to be equipped. Therefore, try to track any smoke that continues to move after an enemy Spy was killed.

When playing as an Engineer, if your Sentry Gun's kill counter does not increase after killing an enemy, he is probably a Spy. Remember that it may be hard to judge if the Spy was using the Dead Ringer since Sentry Guns will generally kill their targets very quickly.

When playing as a Pyro, even when the corpse drops after being ignited, the colored team glow will follow the Spy for a second. Use this to help alert others to the Spy, and to continue to chase and/or kill them. Jarate and Mad Milk also works in a similar way.

As a Pyro, the compression blast is the most useful tool for dealing with Dead Ringer Spies. After igniting a cloaked Dead Ringer Spy, simply push the Spy away from any ammo sources until the Cloak runs out.

By pressing tab, a player can view the scoreboard. In the bottom left of the scoreboard, you can view how many kills you have. You can check your kills after every kill, and if the kill count does not go up after "killing" someone, there is a Dead Ringer Spy around.

Dealing with the Cloak and Dagger

The Spy's Cloak and Dagger allows him to stay cloaked indefinitely, and to hide in unconventional places while waiting for the right time to strike. An area that may seem impossible for a Spy to hide in may be sheltering a particularly cunning Spy who knows where the enemy team will not check for Spies.

In general, most Cloak and Dagger Spies will try to hide in corners in order to avoid being bumped into as they recharge their Cloak. Try to occasionally fire into corners or vantage points with wide fields of view.

One brute-force method for dealing with the Cloak and Dagger is to simply repeatedly check an area where the Spy is hiding. As the Cloak and Dagger has an extremely short charge meter, the Spy cannot make too many movements or their Cloak will run out.

Some unconventional Spies will hide out in the open rather than against walls or in corners. This allows them to maneuver easily to avoid detection, and easily avoids the most often checked areas of a map. Try to keep this in mind if a Spy is suspected to be in the area.

Dealing with Your Eternal Reward

While Spies using Your Eternal Reward cannot use their Disguise Kit at will, they make up for this by being able to quietly Backstabbing any enemies and leaving no body for their teammates to find. A Spy using Your Eternal Reward can be impossible to detect even when he is killing teammates nearby.

Because they cannot use their Disguise Kit, simply identifying the last player that the Spy Backstabbed will let teammates know which player and class may be the Spy. Note that Spies with Your Eternal Reward equipped can still change their disguise weapon.

If backstabbed by the Spy, clearly announce this to teammates, as well as the class that the Spy is now disguised as, to make sure that they know who the Spy is disguised as.

The Spy's victims make no sound and leave no body behind to discover, but the body is still visible for a short time before disappearing. If players suddenly die noiselessly in the distance, it means that the Spy is there, so make sure to warn players as needed.

When the Spy Backstabs his victim, the faint sound of the melee hit can be heard if close enough. This can serve as a key warning that a Spy might be killing teammates in the immediate vicinity.

Remember that any kills made using Your Eternal Reward will not generate kill reports for the opposite team. Pay very close attention to teammates' movements and listen for any melee hit sounds nearby.

When a Spy with the Eternal Reward backstabs a teammate, the teammate's body disappears. However, their weapon remains sitting on the ground. If you see one of your teammates and their weapon on the ground, it's not a bad idea to Spy check him.