[Xenod]

There's no hard and fast rule as to what you should do when you get raised for the first time by an opponent you have no reads on. Obviously, the answer isn't "it depeneds on how he's playing" because you don't have a clue for whatever reason. What you need to do is go by what you DO know instead of obsessing with what you don't.

1) Your cards. Do you have a mid-pp that will usually miss the flop? Do you have a good drawing hand like QJh that can live or die based on a flop? The better a flop can define YOUR hand, then the more likely you should be to call. That way, if the flop gives you great information about the potential of your hand against random hands it makes knowing about his hand less important.

Lets take 2 hands with the same flop. QJh and 99 with a flop of Q52. 99 doesn't learn much with that flop. He's ahead of most random hands still, but is way behind Qx. On the other hand QJh has a pretty good idea that he's ahead, but could still lay the hand down.

2) What are the stack sizes? If your stack dwarfs his, certainly you're more willing to put him all-in. If his stack dwarfs yours then you're reduced to push or fold. If your stack is small relative to the blinds it also steers your decision to push or fold rather than call.