Hehe, when I don't take my opponent seriously I don't mind typing long messages (if they try to start a conversation first). I can type ~80 WPM in long tests and so short messages easily exceed 100 WPM
It's very rare to encounter a cheater online for me, I think most people are paranoid :)
My big clue is when going over the game I can't find one move I could offer an improvement for in any way. Even very strong humans will play practical moves like exchanging when ahead (and not going for that forced mate in 10) or 2nd best moves with tactical poison. When I play over the game afterwards and it seems "perfect" I'll put a few positions into my engine to see.
I use an engine to test because if they're legitimately 400-500 points stronger than me, it will seem like an invincible player anyway.
Sometimes the opening tips me off. If they play 10-15 moves of book in blitz over and over it's a little suspicious to me. Not that strong players aren't booked up, but blitz isn't usually the venue to trot out your theory.
Over the years I've learned that a lot of players use computer programs to interface with websites like this. So, how do you know if your playing a computer or a person?!
In a 3 minute game if the other player initially takes long periods of time for simple moves. This is the first sign. The second, is when in the last 20 seconds, they suddenly can move very quickly. Every move being perfect the game ending with fractional time left. Just like all those computers we play. This is the reason I don't play longer games as it's to hard to see.
The ability to type a novel while playing is also suspicious.
I would love to hear other give aways you all have discovered and I think a list of suspected players would be in order ;)
Just played one, twice...