How can you tell if these are pure breed golden retrievers?

If the dam was bred through stud, that's more reason why she should find the papers. There is no possible way to know if they're purebred if they don't have papers. Ask her who she bred the dam with. Find the breeder and ask for the sire's papers/info. It's not really a big deal whether it's totally purebred, but if you're paying a premium for it (which you are if they say that they're purebred), then you should know that it really is.