It's very hard, since they are so big to begin with! If your's have a gravid spot visible, you can see how dark that is. If you start seeing eyes, they are close. Otherwise, you basically have to see how they look right before they drop a few times, and then you'll know.
With my balloon mollies i bought mine that were pregnant. If they are yellow or white like mine they are really easy to spot. According to how big they are and how dark the spot is, thats a sign that she will give them really realy soon