Platies that are pregnant would have a gravid spot, which is a dark area near their anal fin. They also get fatter as their pregnancy moves along. Sometimes, if you look really closer you can actually see the fry's tiny eyes pressing against the mother's belly.
Thanks for the tip! I've also read the pinned note at the top of our forum. Your tip made it a lot easier. Is the legnth of their pregnacy simmilar to the Guppies?
Yes. Platties have fry about every 28 days. here is a pic of some pregnant fish to help you. Some of the fish get that big they look like they need wheels instead of fins.