A GOOD player will take some time off of the clock even when they have nothing. This keeps the other players guessing at all times. Sometimes it will even frustrate an experienced or frequent player causing them to make a hasty decision. If you see someone checking quickly hand after hand, play them aggressively, unless of course you know the player and they are using this as a method to fool their opponents.

Some veteran players will try to use the clock as a "timing" tool to their advantage. For example, a player may take the full "time out" before they make their decision to check/fold/raise. If they use a large amount of time to simply check, odds are they have nothing. Slightly raise the stakes to keep them in check. If they come back with a large raise or an all-in raise, watch out! Unless you know this player very well it is wise to be cautious and back down. Regardless of the outcome use it to your advantage by taking mental note of the player's style and use this knowledge for the next showdown. A novice player will consistently play their hand the same way every time.

I'll give you a good example. I once knew a player that went all-in every time he would stay in a hand. It didn't matter if it was on the flop, the turn, or the river, if he bet it would be all-in. Believe it or not, he won a lot of tournaments until I (and eventually a few others) caught up to him by carefully observing his playing style. He would only play Ace/King, Ace/Ace, or any suited cards. Once this tell was out, it was easy to know when to stay in a pot or when to fold. My point is this: If you will carefully watch a player and take note of what cards they hold when they win AND loose, you can get a good feel of how they play and thus recognize their "tells".