NEED HELP!! How can I tell if a baby rat is ready to leave it's mother?

I want to take two baby rats home from a place that uses them to feed snakes, but I'm not sure how old they are, or if they're inbred. How can I get a good idea of how old they are and if I can take them away from the mother? I also don't see them 24/7 so I'm not sure if they're always nursing off the mother, or nursing and eating regular food. I have seen them nurse though so I do know that they are still nursing, but their eyes are open, and they're moving around fairly well, so I'm thinking they're about 3 weeks old, but i'm not totally sure. Would it be a good idea to take them away yet? Because I'm afraid that if I don't do it soon that they'll get fed to a snake :( also will the fact that they might be inbred be a problem too? please help me! and thanks if you do :)