How can I tell my real cat from the doppleganger?!

Well, there are any number of things that you could do to distinguish one cat from the other, but since you don’t know which one is Buster to start with, what exactly would be the point?

By the way, “tabby” is itself a pattern, not a body type.

Hey! Maybe there’s only one cat and it’s just been living at both houses. Have you ever seen them side by side?

Well, if you can’t tell them apart, then just take the one that comes in when you open the door and assume it’s Buster.

(Just kidding! Both of our cats have identifying numbers tattooed in their ears - this might be your best solution.)

Follow one of the rules of science fiction:
Shoot one of them and the other one will turn out to be your cat.