Females have a gravid spot which is a dark spot near their vent. As her young grow, her abdomen will become larger and larger. When you can see a large dark mass near her vent and she appears ready to pop, she is nearing delivering. A female ready to release fry may appear afraid and hide in the plants or dart about the tank. Place her in a breeding box or a small tank by herself with lots of floating and other live plants.
Stores sell a number of plastic breeding boxes for guppies.
In breeding boxes, the newborns fall below a grate so that the female cannot eat them. Alternatively, provide a lot of plant cover and some fry should survive by hiding. Remove the female to either her normal tank or a small quiet tank to herself after she has finished delivering. Do not keep her in a breeding box for long since the stress can kill her. Do not put her in a breeding box too early or she can have premature birth where many or all fry will die.