It depends on the site and how they generate their random numbers. The bitcoin gambling foundation has always said that to be a real provably fair system, it must have this formula,
Code:
[Server Seed Hash] + [Client Seed] + [Nonce]
Without which it will not be provably fair. Hence they say if that is the formula for generating random numbers then you can trust that casino.
However, thing is that is pretty much the basics of it. I have not read anyone that would have criticized this here on the forum (or probably I missed it). In a much more deeper level of understanding, there would be some aspects of this that can still be cheated. But I can't really say how that would be, but likely some good crafty programmer can think of ways to cheat this if they wanted to.