Now before you say "OMGZ! ANOTHER AIMBOT THREAD?!" I feel this deserves a thread of it's own because it's about a guide to identifying them. Spectators and admins seem to be held as the best counter-measures for aimbotters, but admins aren't always around, or might not have a proper idea of what an aimbot looks like.
What I wanted to do is give some hints and tips for telling whether they're using an aimbot or not, so that others may identify them rather than pressuming they're just very good. Also, you can add to my guide by submitting the ways in which you find them.
Too often do I find myself the only one aware that some guy is aimbotting. The first tip I can give is:
Watch for inconsistency (things that don't make sense in their context). If you're speccing some guy and half the time he acts like a noob moving his mouse very slowly then suddenly his aim snaps onto some target, he's very likely using an aimbot. If he notices you speccing and suddenly he's no longer scoring the huge amount of kills he was before, then he's probably turned it off. These people are usually morons though and will easily slip up, giving you no doubt about his aimbot-usage.
People who use aimbots get so used to it, that you might see them shooting forward without moving the mouse for no particular reason. This is because they are expecting the aim to move toward their target, but their target hasn't arrived yet.
Another tell-tale sign is seeing the aim snap to targets that are already dead. This can panic them as they may be in mid-fight and can't stop the bot aiming at the floor. This is because the aimbot still registers that corpse as 'in use' while the dead players is viewing his death before respawning. Non-aimbotters don't have this trouble.
Even people who are very good at aiming miss shots. This just because they don't know exactly where the alien is going to go all the time. If you see the aim snaps firmly to a target without missing one shot- it's very likely he's botting.
Sometimes it's hard to see how fast their view changes and such, when you're not already spectating them. For this I can only suggest you be curious about their actions. If they behave strangely, or aren't wearing a combination of equipment that you'd expect a person scoring that highly wear- be suspicious. For example, I was just now playing human where I saw something strange. The guy who was on the top of the scoring list was going out with a lasgun and a battlesuit. This is suspicious because the dretches that he was picking off would only cause him trouble if he didn't know how to dodge and position himself well. An experienced player would take lasgun + light armour + helmet + Battery pack to ensure that he has plenty of ammo with which to defend himself. This guy on the other hand just wanted to act as a turret so he opted for more armour. Maybe he just hadn't played the game long enough to know this. If this is so, all the more reason to be suspicious of his high-score.
Generally, if a person is scoring well, that doesn't make him an aimbot. If he's scoring incredibly well, that gives you reason to be suspicious. That's when you spectate him to make sure. Don't accuse him straight off, without spectating first. If he turns out not to be an aimbotter, you'll just end up inflating his ego. =)