I'm not sure if this will help you at all. The woman at the pet store told me that you can usually tell after about 6 months when the fish has had time to develop a little more. When they are juveniles you cannot tell fully if they are male or female.

From my personal experience with mollies, I have been able to notice right around the 4-6 month age range. Also, if you look on the bottom of the fish you have now, the female mollie will have more of a fan like fin rather than a tube shaped fin like the male. The fan will look like one of those asian hand held fans and will be rounder in shape like a butterfly wing. Hope this helps you.

Good luck!