How can you tell if a snake is a boy or a girl?

Unless you plan on breeding them, there's no reason to, just pick a gender to call it.

If you really need to know, there are several methods. The only method I would recommend without veterinary assistance is not very accurate unless you've had allot of practice, and that is to compare then length of the tail past the cloaca, males usually have longer, more slender tails. The other methods are 'popping' and 'probing', but unless you are experienced this is best left to a breeder or vet.