Decent AC specialists will use a system whereby when the ac is regassed (560g for the Golf IIRC) it will also put into the system some tracer dye. Technically if there is a leak in the pipework this should be picked up by the pressure testing they do prior to the recharge. A faulty compressor will usually give evidence of tracer dye (seen under UV light) close the compressor unit, but depends on type of failure. Does engine idle change at all when the AC is on and car is stationary, over standard tick over. Pulleys can sometimes be known to slip on the compressor, or have clutch failures. I'm pretty sure that physical/hardware AC issues are covered under the used warranty.