How do I sex a baby WTF?

[AmphibianAwesomeness]

I recently got two WFTs from Denny's pet world in kirkland. I think Charlie is a male because he has markings underneath his armpits and he is fairly smaller then my other frog, Bertha. It may just be because one is younger than the other.
ANY HELP WOULD BE MUCH APPRECIATED!

[AmphibianAwesomeness]

I just played a frog call and bertha started croaking. charlie didn't.

[AmphibianAwesomeness]

Ok, think I got my answer. I woke up this morning and saw that bertha [the little one that did not start croaking] was on the glass. And charlie [the big one that started croaking] was beneath bertha. I think that charlie is a bit older than bertha. I will know once they get full grown because bertha will be bigger than him.