Spies can't be seen, but they can be heard. Watch the kill/death list on the top right and listen to any screams from a teammate. If one screams out loud, quickly turn around and check to see if it's a Spy who backstabbed. The prime targets for a Spy are Engineer, Sniper, Heavy, Medic and basically anyone who is helping the team advance or defend.;
, Spies have the ability to disguise as a class but the name of the player will always be random. If you notice someone running around with your name, notice you and flee, give chase and start firing. If a teammate is doing something that's suspicious, tail him/her and see what happens. If the person can sap a building, it's a Spy.
 As said before, Spies like to play someone else and it's only a matter of time before someone your team mixes up the fake player with the real one. Always try to damage your Medic before going with the flow of the game. Spies can easily trick you and stab you while you're running. Remember, spies can't heal so it's easy to see who is a spy.
 A tactic everyone is familiar with, shoot everyone and anyone on your team. If the teammate takes damage, that player may be a Spy. Fire damage is the most popular to exploit for Spy-checking. Another way to Spy check is to play a Scout and run into everyone. Only teammates can phase through teammates, while enemies will run into each other and become roadblocks. Another way is to check the speed. Scouts and Medics are the only fastest classes past the Engineer, Sniper, Pyro and Spy. If you see another Scout that's running at their speed, it's a Spy. If you see a Medic who's running are their speed, it's a Spy. Force-a-Natures also provide a unique Spy check by pushing someone away. Use the two shots in one go and you can kill a Spy. One final way is to use explosive knockback by grenades or rockets.
Watch and listen.
Keep an eye on every teammate.
Check the Medic.
Spy check.
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