they can give birth to fry about every 28 days. you will notice a gravity spot (dark brown or black area) in their belly region. about a week before birth they will "square off" - will look like their is a box in her belly. and she will get huge and look like she is going to explode. (all of this is less noticeable the first few batches of fry, but the more batches she has the more noticeable it will be, and each time she will have more fry) then 1-12hrs before she starts popping out fry she will poop white. then it can take them up to 24 hours to give birth to all their fry.
once they are finished they will poop normally again, and will start eating their own fry if not separated.
also im pretty sure that mollies and platies are just like guppies, they can store sperm and have up to 5 batches of fry even if they were only impregnated once.
hope this helps answer your question. if you want to know anything else just ask