Originally Posted by juiceeQ
The main thing is to pay attention. Pay attention to betting patterns, and what they show down at the end of the hand (if they do). If they're involved in a hand, and end up mucking at showdown, take a look at the hand history and see what they mucked to get an idea of types of hands they may be playing with, and the type of betting they're doing. There's really no set behaviors to look for, IMO, as everyone/every situation is different.
Another thing to keep in mind is that you're not trying to figure out their exact hands though, the thing that most players do is decide a players RANGE (Such as 77+, AJs+) and then figuring out if anything in that range is beating you.
All the action in the hand can help you narrow down ranges on opponents, and by paying attention, it can benefit you by learning how loose/tight/passive/aggressive they are, which only further narrows the ranges.