I'm presuming we are talking about
online poker? Unfortunately, unlike live
poker games, you can't watch your opponents body language or face to try and pick up a tell. But do not despair, how you bet in poker is as much a tell as a twitching eye.
The key thing is to keep an eye on how your opponent bets when he has a strong hand and how he bets when he has a weak hand or missed the flop. You can't just assume that anyone with a strong hand will bet big. Some players try to trap often while others will stick to "normal" betting patterns.
Basically if an opponent normally makes 50% pot size bets when he has a strong hand you can assume that when he bets 50% of the pot that he indeed has a strong hand. Of course he may be semi-
bluffing with AK or something similar so that's why it's important to watch the players at your table.
Often times players also make "blocking" bets. These tend to be small bets with which they try to prevent someone else from making a big bet before them. These players are almost always on a draw and are trying to see the next card cheap. If you have a strong hand do not allow this. If you are presented with a blocking bet then raise it up to 80% or more of the pot and in most cases they will fold.
Picking up betting patterns is your most valuable tool in online poker and it will take a while to master. When you do master it however you will always have an advantage. So to recap, keep your eyes open to see what cards your opponents played and how they made their bets when they had those cards. Make notes to help you keep track of their betting patterns and
bluff patterns and you'll know where you stand most of the time.
Be aware though that very good players will often vary their bets deliberately in order to prevent opponents getting a read on them. Sometimes they'll bet small when they have a monster and bet big when they have nothing just to confuse the issue. Against such players take every bet seriously.
If you're not sure that you have the best hand and you can't tell if your opponent has what he says he has then assume he is telling the truth until you can get a read on him.
As a beginner you should also stick to premium opening hands in order to make post flop play easier on yourself. If you know you have the best hand on the flop it rarely matters whether your opponent has a good hand or not.