If the charge ports haven't been changed then it probably has R12 in the system but the only way to know for sure would be to have an A/C shop with a tester identify the refrigerant.

If it has 134A in the system, legally it would need to be removed with a 134A recycling machine.

Flushing the system would be required to remove the PAG oil if going from 134A back to R12 and wouldn't be a bad idea even if the system has R12 in it. I would replace the dryer also.

I prefer to use R12 in my old cars, fortunately I still have some left. I would suggest sticking with R12 if you have access to it.