For guppies I know they have a black spot on their belly but the female should get real plump and when she's about ready to have her fry she should look like she's gona explode I believe. If i'm wrong I'm sure somebody will correct me.
if you can see through to thier bellies you will be able to see the fry eyes-- black spots that grow as she becomes ready to let them go--also definatelt you will see thier bellies grow out and downward-- really they do look like they are going to blow--you'll know!!
