I think the cloaca is a clear sign.
I´ve seen some females with dark hands and a few adult males without noticeable nuptial pads, but looking at the cloaca, it was always possible to determine the frog´s sex (as long as they´re old enough).

P.S. also my male started croaking by the time his arms became black; if you didn´t hear the albino by now, I think that´s another hint