I have a 91 camry that failed the CA smog test because it lets out a puff of smoke when you floor and release the throttle. Could it be anything other than the rings?
Yes. Could be valve stem seals, but unlikely as they usually show smoke on start up.
Rings show smoke on acceleration, valve stem seals on deceleration when cylinder vacuum is high…and in the morning after oil has had a chance to weep down onto the valve backs and into any cylinder with the valve open. Revving the engine this way creates both acceleration and deceleration conditions, so either or both could be shot. On a 20 year old Camry I’d suspect the engine is just shot. I’d bet on it. You can verify it with a simple compression check.