R12 or R134a

Does your air conditioning blow cold at all? I think an AC shop may have equipment to tell what type the charge is.

The best way to service the unit is to evacuate the charge into a holding tank, weigh how much refrigerant was removed. Wait for a period to confirm that the system holds the vacuum. If not, locate the leak. Recharge based upon weight vs. just relying on pressure.

Quite often in cars our age the Schrader valves are bad, the hoses may leak, and the compressor seals may be leaking. I've had to replace the compressor, hoses and expansion valve on my 1987 928 S4. The AC gets a good workout in the Arizona heat.

If you plan to charge the system with R134A, it may be a good time to change the hoses since R314A molecules are smaller and will leak faster.

Since the hose is the stock hose, you cannot tell what refrigerant is inside. Later model cars that come with 134A will have 134A only connectors.

If the system was upgraded by a professional it should have an upgrade sticker. If not, you just don't know unless a test is performed.