Don't be fooled by people checking. Usually it's because they don't have a decent hand and want to take a look at the next card for free in the hope that they make a hand, however it can also be a "slow-play". This is where the player is trying to hide the fact that he's got a good hand in the hope of getting somebody else to start the betting so beware.
Raising after checking (check raise)
Even more dangerous than just checking. If you get "check-raised" you need to make sure that you've got a VERY good hand, because your opponent certainly has!
Your opponent is saying to you "I've put all of my chips in on this one, do you feel lucky punk?" It may be a stone sold bluff but he has acted first so you must have a hand you feel sure is winning to call him, or else you must fold.
Smarter poker players know that if they have a great hand they must do their best to extract the maximum amount of cash from the rest of the table. Sometimes a small raise is a sign of this. The player wants to get the betting going to fill up the pot but doesn't want to make the bet so high that it will scare everybody off.
Long delay before check or call
A long delay can indicate a less experienced player who has just landed a massive hand. He's not sure whether to check, call or raise and thinks about it for a while before deciding just to check to see what happens.
Ace or king on the flop
If you don't have an Ace or King, and there are three or more players in the flop, this is a very bad flop for you. There's a very good chance that someone else is holding one. Unless you've hit two pairs or triples, or are on a flush or straight draw, get out now.
Pair on the flop
Another big danger sign. If anybody's holding a third one in their hand they are in a very strong position. Be sure you have a great hand or get out if you can't just check and see a free card.
Two or Three suited cards on the flop
Danger - flush alert! Anybody holding two of the same suit as the suited cards on the table might come back with a big raise or even an all-in. Do you feel sure enough to be in the betting when that happens?