Gee, come to think about it, I don’t know. I’ve always trusted them, because everyone else does, but I can’t say I know their actual credentials.
They do give their sources, though. That’s usually a sign of having done their homework. Unless those sources don’t really exist (which should be easy enough to check), I think we can trust that what Snopes says is actually from those sources, since it would make little sense to go through the trouble of finding proper sources and then not use them.