I think they can be rigged but unless you have a way of manually checking the roulette wheel you can't say which site is cheating and which are not. These are some of the ways they can be rigged: http://www.winning-roulette.net/cheat.php

But this article might be old and they may very have better ways of doing it. When you're playing at Online Casinos there's always a chance that the house might cheat which is why bitcoin gambling is based on a provably fair system but if a site is not offering any ways of checking the rolls then just stay away from it.