look at them closely or measure them.
I've been getting 15x7 steelies from 5th Ave cars and the spare is almost always 15x6.
There's also a fairly high chance that the spare has been swapped on to the road and no one noticed it wasn't the same size. IE you find three 15x7 and one 15x6 on the car, and the spare may or may not be present in the trunk.