I don't play spy much anymore. Back in TFC, it was probably my favorite class after engineer. It's not that I don't like the class anymore. If anything, I like it more because it now takes a decent amount more skill. Good spies are hard to come by. But even then, a good spy generally isn't THAT much better than a mediocre one. I've seen the worst spies still score kill after kill. It takes a truly clueless individual to not be able to be at least semi-effective with the spy class.

No, I don't play the spy class anymore for the sole reason that my skills are better put into a class that fewer people play.

Now, here I'm going to go into the main reason spies score so well. It's not because they are godlike machines of death. It's much simpler than that. It's because players just are not paying attention to details that are punching them in the face.

Spotting a Spy
Most of this is common sense (which I've found is not so common). Spotting a dumb spy is quite easy, but it's still overlooked so often.

1. Have a good idea where your teammates are:
No, you don't have to keep a note pad, but if [TG]TheFeniX has been talking about his sentry gun down in the flag room for the past 5 minutes and you suddenly see him running around with a wrench upstairs: kill him. He's probably a spy.

2. Know how players operate:
Spies (when disguised) show the main weapon of the character class they are disguised as. The only exceptions are the Engineer and Medic (oh, and engineers also need to stop running around with their wrench out unless they are repairing. You have a shotgun for a reason). I'm pretty sure these are the only exceptions, but feel free to correct me.

Spotting Engineer spies is a lot easier, but other classes can be easy to spot with a little sense. Does the soldier on your team like to roll with his shotgun out? Guess what: if you see a soldier with a shotgun, he's not a spy. Same with any other class rolling with their secondary weapon.

Medic with his medi-gun out not healing? Blast him.

3. Know how the game works
Players can walk through their own teammates. Enemies literally slide off each other. If you run into a teammate are you're forced to "push" by him, blast him. He's a spy. This seems self evident, but it goes unnoticed so much. Hell, I've noticed it when I wasn't even the one "bumped" into.

Spies cannot shoot their weapon and stay disguised. So, if you see a bunch of players blasting away at each other and one soldier isn't firing or reloading, he's probably a spy. Shoot him to make sure.

4. Communicate:
Prove you're not a spy: "I'm not a spy" works just fine, but you can also just shoot your gun.

5. Watch your back:
Not just for spies mind you, but when you see a medic following you and he's not healing you... well... shoot him.

6. Be Observant:
This is technically already covered, but it can't be stressed enough. Pay attention to the little things that look out of place. DO NOT just blow them off. Did you walk around a corner and kind of get "bumped" for no reason? Sure, it could be the netcode, but it could also be a cloaked spy you bumped into.

Look for teammates just not acting "right" or in the wrong place.

I'm not saying I'm the master of spy killing, but I do tend to kill a lot more spies (or at least see them coming before getting killed) than they kill me. Just keep your eyes open guys. And it also helps to have a good count on how many spies the enemy team is fielding.