What are the telltale signs ... ?

... of a cheater?

Since I broke 2000 on the Online Chess and have been playing more in the World League, I seem to be coming across cheaters much more frequently.  Several times, I've seen opponents banned in the middle of games, or I've looked back and seen they've been banned a few weeks after we've played.

So, what are the telltale signs of your opponent cheating?  I guess if they have 200+ Online games going at once and are blitzing every move back within minutes at a very high level, that's probably a bit of a giveaway, but anything else?  What about in Live Chess?  Are there different signs there?

Just want to add that so far I've never reported an opponent for cheating.  Maybe I'm a bit naive?