With the A/C turned OFF (and no windshield defrost turned on either), start the engine and look at the compressor pulley. You'll see that the outer ring is turning, but the center part isn't. Then, turn on the A/C. You should hear a definite "click" and a slight correction in engine idle speed. You should also see the inside part of the compressor pulley is turning along with the outer part. If the compressor only stays engaged for a few seconds and then disengages (engage - disengage - engage - disengage .... etc), then likely the system is low on charge.