While you have your chainring off, use a compass to go around the half of the chainring with the arm forward (the half that the chain is engaged on). Then do the same with the arm facing back. A lesser number will tell you that the ring is the problem.

I had thought about using a diameter measurement, but it doesn't seem to work right in my head. I think an arc measurement would be better. If you can't measure the arc properly (get a length figure), then do the longer one first (arm forward) and attempt to 'overlay' the same arc with the loose side (arm backward) - if they're not on top of each other, or even close, you know it's the ring.