Watch their eyes to see if there's a surprised and happy look or if they immediately look down at their stack to see if they can afford a big wager from here forward. Watch where they tuck the cards. Some players put a chip or lucky charm on top of their cards if they plan to play the hand but just hold a finger or two on them if they plan to fold. Check their head movement. If so, they might be hoping to drag someone into an all-in pot or force someone to fold.
Poker is an easy game to learn.
In fact, you can pick up the rudiments within a few hours. It's one lesson after another, like life. In learning the new lessons, you can't forget the old ones. And one more note. If someone you play against regularly ignores the lessons of poker, don't teach him or her anything.