It doesn't take a lot of time or experience to diagnose a failed or damaged compressor. You can both see and smell signs of damage to the compressor. You will know you have a problem when you get hot air instead of cool air from the air conditioning system. Inspect the compressor's belts and wiring with the engine off for safety. However, you will also need to inspect the compressor's clutch hub with the engine running to ensure that the clutch hub engages.
Locate the compressor behind the radiator of your vehicle's air conditioning system. With the engine off, inspect the compressor for oil, paint damage or rust which will indicate you need to replace the compressor unit.
Check the condition of the drive belt attached to the compressor, near the clutch. Ensure that the drive belt is secured tightly and has no damage.
Turn the clutch hub -- the round part which sticks out on the compressor -- to make sure it rotates with minimal resistance.
Take a look at the wiring around the compressor. If you notice the wiring is burnt or shows other signs of overheating, then you will have to repair the wiring.
Start the engine, and turn on the air conditioning to the highest cool setting. Allow engine to idle with the air conditioner on.
Check the clutch hub while the engine is running; ensure that the hub engages and spins at belt speed. Also, check the drive belt. Be sure that the belt does not slip while the unit is in operation.
Test the temperature of the hoses. If you notice overheating while testing with the engine on, then you will need to replace the hoses. Also, check for refrigerant leaks.
Use the specifications set by the manufacturer of the vehicle when replacing parts. Especially, the drive belt and hoses will need to match the manufacturer's recommendation. Complete service to the air conditioning system every two years for best performance.
Take safety precautions while inspecting the compressor, belts, and hoses with the engine on. Prevent the inhalation of gases by leaving the area well-vented.