The ability to read hands may be the most important weapon a online poker player can have. As the Fundamental Theorem of online Poker suggests, the key mistake in online poker is to play your hand differently from the way you would play it if you knew what your opponent had. The more often you play your hand correctly on the basis of what your opponent has the less you give up and the more you gain. If you somehow knew what your opponent had every time, you almost couldn't lose because you would always play correctly. It follows, then, that the better you are at reading your opponents' hands, the closer you come to perfect play, and the closer you come to perfect play, the less you lose and the more you win.
Reading hands is both an art and a science. It is an art because you must know your opponents. Before you can technically analyze what your opponents might have, you must have played with them for a considerable length of time, seen how they play their hands against you, and most importantly, watched them play hands in which you are not involved. Even when you are not in a hand, you should not relax your concentration. You want to discover how your opponents tend to play the various hands they might have. Will a particular opponent raise with strong hands in early position, or will he slow play? Will he raise on a draw? How does he play his big hands from one round of betting to the next? How often does he bluff? The more you know about an opponent's general playing habits, the less difficulty you will have reading what he might be holding in a specific situation.
Ironically, it is not as hard to read good players as it is to read a bunch of incompetents. When a good online poker player makes a play, there is a sensible reason for it, and your job is to find the reason and put that player on a hand. But there is no pattern to the play of a weak player, and so you must do a great deal of tentative guesswork to put him on a hand. Nevertheless, by playing solidly against weak, unpredictable players, you have to win eventually. Sooner or later a sound, logical online poker player must beat someone playing online poker game by the seat of his pants. The latter may get lucky for a while, catching the inside straights he draws to, winning with two small pair when you raised with aces on third street, but percentages are bound to catch up with him. Many good players get upset when a sucker draws out on them. While it's never pleasant to lose a pot you were favored to win, you should nevertheless welcome these beats. Congratulate such players on hanging in there to make their hands. Encourage them so they play even more sloppily. It shouldn't be long before you have their money.
The more you play against average to good online poker players, the easier it becomes to read your opponents' hands because they tend to check, bet, and raise for logical reasons and with a certain consistency to their play. However, as your opponents get tougher and tougher, your ability to read hands starts to fall off because tough players disguise their hands and they are sometimes intentionally inconsistent. They make tricky, ambiguous plays like semi-bluffing, like raising with the second-best hand, like slow playing right to the end and then check-raising you. They may even play a hand as it would normally be played, which can sometimes be the most deceptive play of all. In a word, they do all the sorts of things we have been discussing in this online guide. They are trying as hard to deceive you about what they have as you are trying to discover what they have. And of course, you are presumably playing your hands equally hard against them, even as you are trying to read their hands.