Well, based on behavior, I'm pretty sure about the the one being a male. He's the largest and the meanest.
Behavior is the best way I've been able to tell too aside from what I mentioned above, there is no mistaking it when they court too. They get that gill plate flared out about twice as far as the female. I bet your 100%, males are crazy arent they? My biggest male regularly "checks" by 9" female Uaru. :nilly: