I'm not absolutely positive about how they diagnose it. But one time my RB tiger had a lung x-ray and while we were looking at the film, the vet assured me that the cat did not have arthritis (which I suspected at the time.) She said she could tell by looking at the joints.

The x-ray was expensive, but the good news is that it's quick and doesn't hurt!

Good luck to your kitty. Hope he's better soon.