One of the key skills most good poker players have is the ability to read their opponents at the table. That is why you hear so much about “poker tells.” A “tell” is any physical reaction, kind of behavior, or habit that gives (or tells) the other players information about your hand. If you learn the most common tells, you can not only watch your own behavior to make sure your body language isn’t telling all your secrets, but also watch for the habits and tics in the poker players you’re at the table with. If you can accurately read your opponent’s tells, you’ll make the right decisions against them more often and win more money.
• Watch the Eyes
This is why many pros wear sunglasses or visors/caps when playing, they know that the eyes rarely lie. For example, many players can't help but stare at big hole cards, so their length of time peeking is longer. Conversely, if a player is looking to steal the pot, he may look to his left to see if the remaining players, who haven't yet acted, have quickly glanced at their cards and are likely to fold. Another example, a player may try to ask you questions about your hand, knowing that people rarely can "look someone straight in the eyes" while being dishonest.
• Repetitive Betting Patterns
Usually the most revealing poker tells are based on the way a player habitually bets during particular situations. For example, maybe the player always checks when he has made the nuts, or a player may regularly fold after being re-raised.
Anxiety typically occurs in people when they are confronted, or anticipating confrontation. Psychologists call this the "Fight or Flight" stimulus response, which links back to the days when we were cavemen/cavewomen. Physical changes happen including flexing of muscles, eye pupil dilation, palpitating heart rate, dry throat. In poker, when someone has a big hand they are typically ready for confrontation and can exhibit some of these characteristics. You may see the chest expanding abnormally, or you may notice the players voice become slightly higher as he makes a comment.
• Weak is Strong/Strong is Weak
In its most simplistic form, this usually applies to novice players but frequently applies to pros as well. Basically, players like to be actors, and when they have a monster hand they tend to look disinterested. For example: "Oh, is it my turn to act" or "Oh I guess I will play these cards". Alternatively, a player increasing the level of his voice while raising the pot, and trying to look intimidating.. may be running a bluff.