You can determine the efficiency of your unit by comparing the temperature of the air that comes out of the ventilation grille with the temperature of the air that comes back in. A visual inspection of the air conditioner is another way to check if the unit is working properly. Puddles of water or coolant should not be present and may indicate leaks through broken lines or through clogged drain holes. It's also a good idea to listen carefully to the signs that the air conditioner isn't working properly.
Squeaks, squeaking and banging noises are common signs that belts, bearings, or other parts are wearing out.