Mollies don't have a gravid spot. In general if you have bought a female molly she's pregnant!!! Regardless of whether you have a male or not. If you do not have a male she will continue to have fry every 45 - 60 days for about 8 months as female live bearers store sperm! You're about to be over run with fry! Although, stress may cause them to abort a pregnancy so you may have to wait a while. Generally speaking, if they're fat, they're pregnant!
P.S. Your quite sure you have female's, right?
P.P.S. Your tank is not big enough for the Red Tailed Black Shark either, I'd be returning or rehoming it, and what sort of plec' do you have? And your tanks look overstocked to me too, but I could be wrong.