I think everyone ahs pointed out the key factor.
Experience.
It takes a long time. The simple thing is that you can play any hand in any way (yes thats a problem) But, especially at lower stakes players play their hands in a similar fashion. You wont see someone raise you with a big draw very often. They call your bets, then often check the danger card to you, as irish mentions. Similarily, big hands often raise the turn unless they dont like the flop. This is another sign. Basically you have to wonder firstly what a player would expect to play from his position (not always good at lower stakes) and then why would they call/fold/raise you with the hands you would expect them to play from their position.