Check the compressor clutch. The front of the clutch will not spin when it's off, and will when it's engaged. Mine started to go and would only cool for a few minutes before it overheated and disengaged.
All off? If its on defroster A/C will automatically be engaged even if the a/c button isn't on.
Ben
Yeah prehaps there are specs in the factory service manual to check the resistance of the clutch engagement coil. Not real sure haven't been through this issue myself.