Simply look at the front of the compressor and see if everything is turning while engine is running. If the compressor is not engaged, the part at the very front will not be turning.

If the clutch is indeed engaging, feel of the line coming out of the heater valve near the battery. If it is as hot as the line going into the valve then the valve is opening when it shouldn't. If this is the case, the problem could be CCU, heater valve or associated wiring.

Good luck,