What tail type are your babies? If they aren't PK or HMPK, you'll notice that the males start developing longer and longer fins as they get older. They'll also start to flare and go after their siblings, at which point you should separate them out of course.
If they are HMPK or PK....Not sure how to tell the difference
I guess its just something you'd need experience to do, but I'd personally just go by aggression level.
My babies are just over 3 months old and I didn't notice the difference between genders until recently, but aggression started at about 8-9 weeks. So just watch them and separate out the aggressive ones ;)