i assume you are trying to sex a leopard gecko since it says in your sig that your girlfriend has one? if so then males have a hemi-penal bulge and pre-anal pores. here is a quick guide to sexing leos:
male: note the enlarged pre-anal pores along with the presence of bulges at the base of the tail.
female: note that there is still somewhat of a bulge but the pre-anal pores are not enlarged. the cloacal opening curves inward more as opposed to being more flat like that of a male.