Their bellies get really fat and protrude down. It will be very noticeable. Don't worry of you miss this time; mollies are prolific breeders! She will get pregnant again very soon. I had mollies when I first started my aquariums. They had so many babies the other fish were tired of eating them!!!
Since their gestation period is about a month, you should be able to estimate when she will give birth. With a standard Molly, the gravid spot near her vent is usually obscured, but it becomes more distended as date nears. A distended belly is always a good indicator of the due date.