How can you tell

many people said that you can tell sex by just looking is this true? if so what do i look for???

EDIT: Oopsies! I answered this for BPs.

For leos, all I know is that males generally have a bigger bulge at the base of their tales than females.

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.
http://www.mlededee.com/herps/leos/leo_male.jpg


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.
http://www.mlededee.com/herps/leos/leo_female01.jpg