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The orange one is definitely pregnant- a lot farther along than the silver platy. You can tell by the dark gravid spots near the anal fins. The farther along they are, the darker the gravid spot (due to the fry growing inside her- they cast a nice shadow on her skin).
I'm not sure on that, but you will be able to tell she is close when she gets box shaped and when she starts hanging out at the bottom, and is probably looking for a spot to drop the fry.
If you would like to.. You could try putting her in for a couple of days straight and if she doesn't drop any fry in that time, let her out for a couple days.