My platy usually gives birth when it is quiet.. so either at night after I fall asleep or during the day when I am at school.. A weird thing I noticed is my platy usually gives birth when it is raining.. I don't know why maybe it's a weird coincidence...

A good thing to look for is if she stops eating (Not all do though), if she doesn't eat she is most likely in labour or about to go into labour, you can expect fry in the next 24 hours.. Also if she has her tail up and her head down she is most likely about to deliver, this is one way a fish will try and push the babies out.