A (somewhat easy) way to sex your frogs is to listen for them at night. Males often "sing" at night, so if you have one of each species and you hear singing, you probably have a male. Male ACF calls sound metallic (some people say they sound like creaking floorboards or a cricket underwater, I think they sound like croaking.) A female will reply with either a rapping sound (Acceptance) or a slow ticking sound (rejection). ADF sounds sound like a loose filter tube. They go MeeEEEp MeeeEEp MeeeEEEp, and sometimes swish their back flippers (as if the females are going to notice and appreciate the gesture as ADFs are really quite blind). Female ADFs don't make noise.
Both species have things called "Nuptial pads" on their forearms. Male ACFs will have a black line on the inside of their forearms and male ADFs will, indeed, have a white dot on their armpits.
I will give the same warning that the other posters have given: If you haven't already, get the ADF and ACF in separate tanks. ACFs will happily eat their smaller cousins if given the chance. Please give each frog a nice, safe home.