Just out of curiosity, are any of them obviously male? The problem with being able to "tell" that "she is a she" is that they all look female until the anal fin differentiates. I was certain my first platy fry was female until at about 8 - 10 weeks he started developing the longer, skinner anal fin. He's definitely male, and he definitely looked female when he was 2 1/2 weeks old, and 6 weeks old. Then again, my eyes aren't as good as they used to be - maybe with younger, sharper eyes I would have been able to tell at 6 weeks, too. Who knows?
With guppies, the females are identifiable before the anal fin differentiates, because of the gravid spot. Female platys also have the gravid spots, but with mine I can hardly see the gravid spots until they are at least half-way through their pregnancies. However, mine are pretty dark. With paler colored platys, you may be able to see the gravid spot and know it's female.
She is a guppy fry, and he already has a gravid spot, and i only have one guppy fry
so unless they all start out with a gravid spot too than she is more than likely a she, but i guess we will see