I have posted a thread in the pinned topic section specifically to help people judge how far along their own fish may be. It has a picture of the same female every few days from one fry drop to the next along with some commentary about what I saw at each stage. It was my hope that it would guide a new person to guess correctly when their female common livebearer was getting close and to provide adequate cover for the fry. My thread can be found easily by clicking on the link to my molly's progress in my signature area.
I like to use a separate birthing tank when there are fry I want to save and put just the single female in that tank along with some good fry cover.