It's hard to tell if the females are truely gravid in some species of molly because the 'gravid spot' doesn't show through their scales well enough (like on the black mollies). Essentially, if you're keeping males with females, then the females are almost surely pregnant. If you've observed breeding behavior (the male molly will swim with the female, nipping at her underside and flicking at her) then I would guess in the next 32-48 days, you'll have molly fry.
You'll know they're pregnant when they blow up like balloons, and their stomach starts to square out. That is, you'll see a flat area on the bottom of their stomach opposed to having a rounded stomach. the females vent will also drop. That's the definitive sign that the female is going to drop her fry in the next 2-3days (if not sooner).