With our hamsters (all at the RB now ) you could usually tell because the males have such big rear ends . Plus, they were always in their own cage, so it didn't really matter to us.

But I answered yes to your question, but I want to add a caviot that in our situation, it wasn't really a lie, they just didn't know any better, I guess! Milly came to us as Willy. They said she was a male, and for all I know, maybe she is, but she has all the traits of a female Cockatiel, thus the name change. There's an old thread here, somewhere, that we discussed this after I got her/him. Birds are especially difficult to sex if you don't have DNA testing done, and for me it didn't matter, so I never did it. We love her whether she is a Milly or a Willy!!!