I’ve observed that a poker player’s “tells” divide into two categories: the way he bets, and the way he looks. Each gives valuable insight into the person’s psychology, and as the old trope goes, you should always play the man and not the hand.
Deciphering physical tells relies on keen observation of the opponent’s body language. Watch where the eyes go, and the way the shoulders slouch. Sometimes it is simple: an experienced player might handle his chips differently, and may even perform neat little flipping tricks with them (I have yet to master these).
A rookie will fumble with them. The way the opponent bets offers most insight, however. There are no better demonstrations of confidence or fear. How an opponent enters a hand is key. Does he always enter the hand with a gentle call? Or does he always raise when he has a strong hand?
Pay attention, and soon your opponent’s strategy will be clear to you. Be careful in how you look, though. I gave the wrong kind of eye to a bearded gentleman in south Montmartre once, and have never been able to bend my knee fully since.