With live bearers(Mollys, platys, guppies, swordtails...) , you can tell they are pregnant because their abdomen looks larger and rounder. You may also see their gravid spot getting darker. (the spot near the anal fin where the fry grow inside the mother) these signs can also be because of bloat, but female live bearers are pregnant most of the time anyway, so if you can see these signs, it's a good bet she is pregnant. Can you upload a picture?