Lil ME, that works OK for a guppy but most mollies are not transparent, they have color in their skin. That means that the gravid spot cannot be seen. You need to judge a molly by her size and how well squared off she is. A molly can look well along in terms of size merely by being well fed so it really comes down to shape. If you can get a picture up, I can try to compare it to the shapes I have on file for different stages of fry development and may be able to give a half decent estimate.