Hello Sil...
You don't need to examine the fish. That way can be way off the actual time. You just need to pay particular attention to the tank water. Gradually work up to the point you remove and replace most of it every week. When you change it, make the new water a bit warmer than what's already in the tank. I always use a little standard aquarium salt to my Platys, replacement water. A rounded teaspoon for every 5 gallons of treated tap water. The warmer, pure water and salt combination will ease the stress on a pregnant female and encourages her to drop the fry when the water is extremely clean.
B