They are not fixed unless they don't claim they are provably fair. When they claim that they are provably fair, you can check with the use of provably fair verifiers. There are gambling sites that are not provably fair, I suggest that you stay away from them and there are also sites that claim that they are provably fair but in fact, they are not.
This is a good remark. Indeed there are such sites and that's why we shouldn't take any claims at face value, especially when dealing with newly appeared sites. Fortunately, it's not that hard to verify several bets. And reading reviews also helps, because we can't verify every bet (well, we can do that technically, but that would be hell of a game). Some gambling sites have such a good reputation that it seems unnecessary to verify any bets there at all, but of course you can do it from time to time, just in case.