Well the shape of the lower half of the shell near the back legs, may be slightly concave in males, but that usually requires the animal to be of a certain age and to have engaged in copulation quite a few times before its considered conclusive, and the males tail is usually slightly longer, though again this isnt completely conclusive.
You sex turtles by attempting a hemipenis eversion by appling a gentle but firm rolling pressure with forefinger and thumb or by using a probe to check for depth of the hemipenile sheath. This isnt something you should do without being shown how though as you could do the animal some serious harm. The shape of the ventral opening is probably not particularly indicative either.
If I were you, i'd take it to a local reptile pro and get them to show you how to do it safetly. I fear merely giving you a description of how to do it wont help, you really need to see it being done.