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GENERALLY, and I stress Generally... people who bet lightning fast (especially after the flop) Don't want to be called. And typically people who think for longer than they normally think and then make an agressive move have a very strong to nut hand.

Look at it like this. What hand can you have (for the lighning fast post flop bet) where you could bet basically without thinking or even really looking at the flop? Even if I have AA I want to SEE what flopped, I want to THINK for a second about how to play the hand. Are there draws, should I bet more or less, etc. The only time you truly don't care what the flop is, is when you ALREADY planned to bet no matter what comes. And more than half the time that means bluff or semi-bluff (with AK for instance).

The opposite is true of nut hand. BAM! you flop a full house. Now what? "OK, how much should I bet to mask my hand and make some money? If I check will someone else bet for me? Who's in the hand?" All of a sudden you need to think. And that's true for most players. And often, you'll get the think... check... Quick Call or Quick Min Raise (if it's on the flop). On the river it will be a think... bet (a strange amount, whether value or all in). But if someone thinks for a long time and then you bet TPTK on the flop and they quickly call. Watch out! It will be followed by another check/call and then a big bet on the river (or big raise depending on their position). That's a made nut hand all day.

(these examples are for NL ring play. MTT and SnG's have some unique "weather patterns" based on situations. And Limit... Ummm how do I say this? In my version of hell, the Devil meets me at the door and says, "You get to play poker for eternity.... But it's LIMIT! AARRGGGHH!" In other words I don't play limit.)