The only way I've been able to sex them is to let my PROVEN male tell me. When I put a new one in (have tried two times) the male will quickly let me know. Thus far, he's chased and beaten both.
The good thing is, when they are in 'chase mode' they move around the tank without any concern about what else is going on. They are determined to attack. This makes it easy to net them as they zip through the water column. This is a good thing. Otherwise, I'd have to rip apart pounds and pounds of LR to get them out.
Dave