Regarding your main question - reading people takes time and skill, but after a while you start to recognize patterns.
Keep notes, if you play in a poker room that does not have notes move to one that does.

Learn to recognize slowplay. For example you hit TPTK from late position - villian checks - you bet the flop and get called, villian checks - you bet the turn and get called, and then he pushes the river on a blank - this is often a slowplayed set.

In every hand ask yourself what is the opponent holding - dont just play your cards