The front part of the belly bulges and gets the squared or boxed appearance and the "hole" for lack of a better word (I'm no scientist) starts to bulge. Some of my mollies hang out about 2-4 inches from the top by themselves... Others hide in vegetation and try to drop them into the middle/on top of my plants.
I've tried to witness a birth but haven't yet. I have about 150-200 babies of varying sizes swimming around right now. Only the strong survive