The collision system in this game works like this: You can "clip" through your teammates and you can be stopped by an enemy if you try to run against him, so if you run into a teammate and you get "blocked", he is a spy, so shoot him until he dies, even if he doesn't try to react, because that is what most spies do in order to make you think that you are shooting a friend.
So if you suspect that someone is a spy, just try to clip through them...

Every class can spy-check. If you have read the previous section, you know how it works. However, some classes have "special" abilities they can use to spot enemy spies:

Pyro: They can simply "fire" every teammate. If he gets on fire, he's spy, so keep on firing until he dies.

Sniper: They can throw their "Jarate" to see if someone is a spy. If the Sniper throws one on his teammate and he gets wet, he's a spy, shoot him with no mercy.
Suggestion by Skepolo: They can also use the Tribalman's Shiv to bleed out an enemy.

Scout: They can use their "Mad Milk" just like Snipers do, if he gets wet, shoot him.
Suggestion by Skepolo: They can also use the Boston Basher to bleed out an enemy.

Enginner: They have a Wrench called "Southern Hospitality" that makes enemies bleed, so you can use that to your advantage if someone is close to your buildings trying to screw you up. I think you got the idea: "If it bleeds, you can KILL it".

- If you see a teammate near your enemies without getting shot, he's a spy, otherwise the enemies would just shoot him.

- Some Disguised Spies keep looking at their enemies worrying if they get spot, so if you see someone looking at you and going backwards when you try to reach him, he's a spy, he's afraid of you.

- Remember, enemy Spies can disguise as teammate Spies too, so spy-check them as well.

- Spies sap Enginner's buildings while disguised, so if you see someone sapping them, he's a spy.

Suggestion by [Black-Out]arachnid14:
- Friendly spies normally try to get into the enemy base, not into their own base, so if you see a friendly spy that won't go to enemy base, he is probaly a spy.

Suggestion by ComicPuma4954:
- If a Scout is not running as fast as he should, he's a spy.

Suggestion by Warren elite:
When somebody walks around the same place they're probably a spy.

Suggestions by Coolz:
When a spy disguises as a spy, he will ALWAYS be shown with a random disguise on. If you see a friendly spy without any disguises on, he is actually on your side.

Professional spies will never let you know someones even playing as a spy before the round begins. They will disguise as their teammates. If you see any enemy class die faster than they should have (like a pyro runs out of spawn and takes a one damage bullet and instantly dies), they are more than likely a dead ringer spy.

When you spycheck people as pyro, thanks to the spycicle, enemy spies weilding it will not get lit.
They will, however yell out about how they're on fire despite them, well, not being on fire.

Certain spies are smart enough to spam switching weapons since most spies dont even know you can do that and people not suspecting you of being a spy for not knowing its possible-- if you see a teammate just constantly switching weapons they are probably a spy.

I know that this guide doesn't show you ALL the possible ways to identify Spies, but the ones shown here are the most common ones. If you think i should add other ways to spot them, feel free to type them in the comments. Also, rate this guide if it helped you, i like to help people :D

Check out my other guides:
http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=235409983
http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=208338189
http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=422490963
http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=250743733
http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=266383738
http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=256856535