If your mollie is pregnant she should have a dark gravid patch, just to the rear of her belly and will eventually look fatter. However, they can look fatter for a number of other reasons, too, for example constipation, so you can't rely purely on their size.
The males will have a quite visible "gonopod" on their underside. Obviously, the females won't. (A bit like humans really!!!! - without trying to get too graphic.)
Unfortunately, the fry will get eaten, unless you transfer the female to a breeding tank in order to raise the fry.
I can give you more help with this if you want (just ask), but if you are a newcomer, it may be best to get used to normal aquarium husbandry first!!
Hope this helps.