I just had my first fry about 10 days ago from my Sunburst platy. I noticed she had starting to hover by herself behind some plants in the back. Sure enough, I saw a little fry zoom up to the surface just before mom had her for a snack. Once I got her in the breeding trap I was able to save about 20 fry.
If she has that square look to her belly and she starts to hover down low then plan on fry soon.