Don't do anything. If you genially want to be this person's friend, then go for it, but don't try to be friendly to them so you can try to "help them figure out their gender" or anything misguided like that, because you don't know them that way, they could just be a butch woman, non-binary, gender-queer, non-conforming, anything really. It's rude and presumptuous if you try to get them to out themselves when they are not really in.