I always look for the preflowers after a month. It can be a little tricky sometimes as I just had one that I was almost certain was a girl, but turned out male. Look for a more spear looking preflower if they're long and skinny its most likely a female, the males can look sorta like spears too sometimes, but if they're shorter and more round on the bottom they're probably male...its a lot easier if you get on that with a magnifying glass too. hope that helped