Hi,
I keep platies and have been through several births with them, so here is my limited knowledge.
My platies are huge by them time they give birth. If you look at the platy head on it should look square in shape. Also there will be a black spot (called the gravid spot) just below her anal fin. This I believe are the eyes of the fry. My platies will always hide away right at the back of the tank where know no one can see them. They only come out for food and then only briefly. At this point I know they are going to give birth very shortly. However I have read (correct anyone if I am wrong) that platies can hold on to the fry and even re-absorb them if they are stressed.
Personally I do not use breeder nets or fry tanks. I think they cause my platies too much stress, however that is my own opinion. Platies can abort there fry if stressed too much. I use the survival of the fittess technique. My tank has lots of plants and hiding places and a few of the fry do survive.
I feed the fry flake food that I crush up very small but you can by foods eapecially for fry. I have a fry in my tank at the moment, some are 5 weeks and some where born only a few days ago. They are very cute but remember that females can store up to 7 amounts of sperm and therefore can give birth every month. it can make for a lot of fry!!!
I now only keep females, so as soon as I can sex the fry and they are big enough the males go off to my lfs.
Hope this helps a bit. If I were you I would let her out of the net and wait and see what happens. She will have them when she is ready and not before.