I have a 72 Chevelle with factory air. how can I tell if it has been converted from R-12 to 134A?

Thanks,

cw

By the service ports.

can you tell me more?

Take your car into a AC Shop that repairs and refills the units.

R12 fitting will be a 1/4 inch schrader port on the lowside and a 3/16 on the highside. R134 will have a quick connect style hook up. They are much larger than the threaded R12 ports with no threads on them.

It will only have the R-134 quick connect adapters IF they were left on after the conversion.
Of course part of the conversion is supposed to be leaving the adapters on and clearly labeling the system.

When in doubt, evacuate out.