R-134a is the refrigerant found in most automotive AC units today. Procedures for checking the refrigerant level do not vary much from one to the other. The best way to check the refrigerant level (whether it is Freon or R-134a) is to hook up a pressure gauge assembly to the service valves. This tool has two hoses that connect to the high and low sides of the car's AC system. A third hose connects to a recovery or recycling tank.
Remove the dust caps from the AC service valves. With both valves closed on the pressure gauge assembly, connect the red (high pressure) hose to the high-side service valve fitting, which is located between the compressor and the condenser. Be sure the fittings are compatible--R-12 (Freon) service valves have a threaded male fitting while R-134a valves have a threaded female fitting. In most late-model cars, the fittings are of the Schrader type and are spring loaded.
Connect the blue (low-side) hose in the same manner. The low-side AC service valve fitting will be located on the compressor.
Note the reading on the high-pressure gauge without starting the engine. A pressure reading in the near vicinity of 50 psi should indicate that there is sufficient charge. If it is below 50 psi, then the system needs a charge and should not be run.
Check the indicator sight glass (if your AC system has one) for further signs that your AC system might need a charge of refrigerant. If the charge is full, the glass will probably be clear. If the glass is foamy or bubbly, the refrigerant is likely to be low. The same applies to an oil-streaked glass.
