Play enough hands of no-limit hold'em and you start to recognize patterns — everywhere. In your own play, and in the play of your opponents.
Newer players start out focusing more on their own patterns than what others are doing, but eventually they begin to pick up on opponents' play as well — especially with regard to preflop decisions and actions. They'll notice who is raising before the flop and who isn't. Sometimes they'll pay attention to showdowns, too, and start making connections between the hands the preflop raisers are playing and the circumstances of their having raised (position, stack sizes, who is in the blinds, etc.