I pay off

[californicator]

It's really a matter of reading into the texture of the board. If you had raised preflop with AA or KK as you've said, you've at least defined your hand to the other players. Facing an opponent that plays back at you, you've got to determine whether the board is one that facilitates your opponent having hit a set or 2-pair.

On flops like 9 7 4, I'd be more weary of a set. On a flop like J 10 5, I'd be worried about J/10 2-pair possibly, or someone holding KQ.