First, a disclaimer: I'm not a Spy main. That means I don't know how Spies think. However, I am a Medic main, which means I know how Spies act. I've trained myself to be able to spot Spies instantly (just like every other competent TF2 player), and most of the time, I'm only ever backstabbed when I get trickstabbed. (We all love TF2's melee hitreg, don't we?)
However, new players can be easily fooled by a good Spy's disguise, and this includes new Medic mains. So I've compiled a list of notes I've taken to help you find that Spy and turn that douche-baguette into a dead body. I'll be talking mainly from the Medic's point of view here, but some of these tips can work to some degree on other classes, too. So, here we go!
#1: Just...turn around.
I know it's repeated often so much it's become a meme at this point, but it's true. Since a Medic isn't actively dealing damage to the enemy (and therefore doesn't need to face forward as much) and since the healing beam can bend around to stay on your patient even if you can't see them, you can usually turn around for a second or two and scan the area behind you.
Sometimes, you'll find a Spy who just backstabbed one of your teammates or is otherwise undisguised. In that case, just use the "Spy!" command, whip out your crossbow, and hope that your teammates aren't deaf.
But sometimes, you'll see nothing but teammates—and one of those teammates might be a Spy in disguise. In that case, don't trust all of them just yet. In fact...
#2: NEVER trust Scouts and Pyros.
Scout and Pyro are undoubtedly Spy's favorite disguises. He doesn't lose out on his 107% speed with the Scout disguise, and Scout's model is relatively small, meaning he can stay hidden for a little longer. Pyro's speed is just barely slower at 100%, and Pyros aren't expected to fire unless they're right in front of their enemies anyway.
Both are often found near Engineer nests, too: Scouts use the Dispenser because the Medic's keeping the Demo and Soldier alive, and "Pybros" protect Engineer nests by reflecting enemy projectiles and killing Spies.
Finally, Scout and Pyro are easy classes to quickly disguise as if the Spy's not using the concise disguising menu. It's a lot easier to tap 4-1 and 4-3 than it is to tap 4 and take your left hand off WASD or your right hand off the mouse to reach all the way over to the 9 key to disguise as a Spy.
For these reasons and probably more, Spies will often be found disguising as Scouts and Pyros. If you see one running behind you, just shoot him. Most good Scouts and Pyros also know that Spies like to disguise as them and that Medics are Spy-paranoid, so if they realize you're spy-checking them, they'll probably just shoot their Scattergun or puff some flames to clear their name.
#3: Shoot any Scouts who are running as fast as you.
Medics and Spies have the same move speed: 107%. Scouts move at 133% speed. When a Spy takes the disguise of a slower class, he slows down to match the disguise's speed. But when a Spy disguises as a faster class—like the Scout—he can't increase his move speed to match, and his move speed stays at 107%.
This, along with the knowledge that Spies like to disguise as Scouts, means you can tell if a Scout is suspiciously slow easier than other classes can—if a Scout's really a Spy, they'll be moving at the exact same speed you are instead of breezing past you.
One thing that can throw a wrench in this plan, however, is Scout's unlockable Baby Face's Blaster, which slows the Scout's base speed down to 120%. On paper, it should still be a noticeable difference, right?
Well, in practice, your TF2 instincts will most likely only store two possible speed values when you see a Scout: 133% for a legitimate Scout, and 107% for a Spy disguised as a Scout. If the Scout's not moving at that full 133%, then your instincts will err on the side of caution, treating him like a 107% speed Spy instead.
