How can I tell if my mickey mouse plattie is pregnant or just fat she started lying around on the bottom about 15 days ago so I put he in a smaller tank by herself but still nothing has happen she looks like she is going to bust

How is your fish doing?

You can tell a platy is pregnant by the huge belly.

Another thing to look out for if your not sure a fish is pregnant is a disease called Dropsy.
It causes sever bloating of the fish and makes the scales stand out like a pinecone.
If this is the case the fish would need to be treated with medication.

More than likely if the fins are not poking out like a pine cone, the fish is pregnant. Hope all is going well.