How do I measure for the size of an auto wheel rim?

You measure the tire bead diameter, not the rim diameter. If you do measure the rim, the tire size is about 1" less.

Most wheels have the size stamped or marked onto the wheel itself.

Unknown Rims? If they had tires on them, might say.
You may also want to measure the hole in the center and the distance between 2 lugs and same amount of lugs, if diameter was not what you were looking for.