The ability to read the opponents is as important in winning poker as disguising of one's own hand. The skill requires knowledge of human nature and a bit of probability mathematics.
The psychological aspect of poker is to get into the opponents' position. For example, you have
Q-J in the pocket and the flop comes with Q-6-2. You bet in last position. The next card is 2. You bet again, but this time the blind check-raises. Do you call? Best to consider, what kind of hand you would need for a check-raise in blind position. Surely more than the highest pair with a T as a kicker.
Elementary mathematics can help to estimate the probabilities of different outcomes. For example, a paired flop like Q-Q-4 is less probable to help someone than three cards of different rank like Q-9-4, because there are only five cards out instead of the usual nine that can fit the flop.
The Threat of a Flush
One of the most common problems is to estimate the probability of a flush when there are three suited cards on the board. In tight games, where the opponents play strictly according to the pot odds, you have to get back and estimate if the draw got correct pot odds on the betting rounds earlier. What kind of tactics did the opponents use? Did they bet, raise or just call? A bettor or a raiser doesn't usually have a draw. Especially, if the raiser is sitting immediately next to the bettor, he doesn't have a flush, because he raised in order to force out the draws.
In loose games, the probability of a flush can be nearly 40% when there are three cards of the same suit on the board. Also, with many opponents, the pot odds allow betting and sometimes even raising with a draw. In that case you have to look at the order of the cards on the board. If the flop came with only one suited card and the others came on the turn and on the river, a flush is less probable, since a backdoor draw is not playable alone.
The Threat of a Full House
When the board is paired, the threat of a full house is obvious. But how obvious? To settle it, you have to get into your opponents' position again and estimate which pocket hands would have been playable and how they would fit the pair on the board. If there are three high cards on the board (e.g. A-J-2-J), the threat is very real, since A-J is a good starting hand in all positions and unsuited as well. The possibility is much lower when a small or a medium card pairs. Firstly, the pair must be backed up by a connector or a high card to fit a pocket hand. For example 7-7 must be backed up by 6, 8 or of the big cards, at least a Q. Secondly, these starting hands are playable only if suited. Thirdly, only A-7s is strong enough to be played from early position. The other combinations are playable from middle position, except K-7s and Q-7s, which are approved starting hands only from late position. For example, T-7-2-7 is a harmless combination because T-7 is a worthless pocket hand. The blind can of course have whatever, e.g. 7-2. A small blind can also be dangerous. In loose games, many players call a half bet, taking no notice of their cards.
The tells are different online. One of the most reliable tells is the pre-selected checking. When many players check in advance, it sounds like a machine gun. They don't have anything. A bluff from late position is definitely worth a single bet. Consequently, if you are sitting in early position, and the whole field checked on the last betting round, and the next card is lower than the highest card on the board, the winning strategy is to bet as a bluff or semi-bluff.
Betting or raising too fast is also a reliable tell. Why is that player so aggressive? Because he has a very strong hand? Certainly not. In poker, confidence expresses uncertainty.
On the other hand, very slow betting, calling, or checking is not a reliable tell. The reason can be anything whatever outside the game. However, sometimes a player checking slowly is trying to pretend some strength and wanting a free card. A check-raiser never goes slow with his checking.