Have you seen the male mate her? It is very easy to spot. Her stomach will swell, gravid spot will get larger. She will be very wide and her stomach will more-so bow out than get deeper. It is kind of hard to predict and catch the birth but it is about every 45 days and if you notice her hiding or sitting in an upper corner that is a definite sign. Live-bearers eat their young quickly, I have seen my guppy hover in a top corner and as soon as a fry was loose she would dart downward after it to try and eat. With mollies you can sometimes see sets of eyes in her gravid spot, when you think she is very large try separating her into a rearing tank by herself, maybe add a little salt and leave her for a day or two. Make sure wherever the babies are being born they have hiding spots.
Separate the female from the male if there is only one female, he will chase and mate her literally to death or at least miscarriage. The male can harass the female excessively even if she is not pregnant. I am about to separate my mollies because although he is harassing her she does not seem to mind, she is always in very close proximity to him but my male guppy is also mating her and my wife does not want "muppies".
You never know though, she may be big and if you face her head-on she will look square shaped, put her in the maternity and nothing and then one you the tank is full of babies. You may not catch her birthing but if they are uncomfortable or stressed they can "hold" the fry for weeks until conditions are right.
Good luck with the mollie fry and let us know how she does.