Once they get to full size the females will be the ones that don't have noticeable colour (orange or red) into the fins.
Subtler, males get dark around the throat/ chest when in breeding mode and the females have a faint dark stripe down the lateral line.
Have you tried posting photos to see if anyone can tell male/ female?
I've only had one male and he died in apparent good health, I wonder if the male rainbow (tank boss) stressed him to death just from the pheromones. I never saw aggression nor injury though and he seemed to bumble about unperturbed. I have 5 lovely females now.