sorry no one with more experience than me has posted yet... i bet if you give more info they will be more willing to help. what species, what age, post pics if you have a specific frog in mind. i've been inquiring about a citronella tinc of mine and have read and been told that females are typically larger as adults, have a wider girth just above the hips, and more petite toe pads. males are typically more slender, smaller, with larger, almost heart shaped toe pads. hopefully if you post more info more experienced froggers will comment.... good luck!
1) Physical Traits
a. Toe Pad Size
b. General Size and Shape
c. Throat Patch
a. Calling after spraying
b. Aplexus for mating vs. wrestling for territory
I'm sure other people will chime in. Aside from the above, I place a couple frogs together in the same tank. If one starts to get stressed and decreases its appetite I generally will guess that they are the same sex and remove one. If they are both doing well, then I don't really bother them and hope that I see some tadpoles. If not, you rechallenge them as above - rinse, wash, and repeat.
Like I said, I think you'll hear other folks chime in and they will have better recommendations than me.
can you see it other times or just after calling? I know if they call they are automatically males so that would only help me out if you can see it other times as well. i have a 10 month old and two 13 month old citronella tincs that I think are males but haven't called yet