First off, do you mean Southern or Northern Leopard frogs? Both are similar, but it would help if we knew which you were talking about.

From what I know on sexing ranids, if the "eardrum" or typanum is larger than the eye, it typically means it is a male. Also, males will often be smaller than females, and have darker, or yellow, throats. However, I am not sure if either applies to leopard frogs. Males will also croak.

The best time to sex frogs, though, will be during their breeding season.

I'll go ahead and do a bit of research for you.

Update:

[Quote from this page: http://www.newton.dep.anl.gov/askasc...0/zoo00230.htm]

"It is not easy! Males are a little smaller than females and have stouter front legs with enlarged "thumbs." The vocal sac or loose skin around the vocal sac of the male may be visible at the edge of the throat during breeding season."