Jim there should be a sight glass in the system ,with the compressor running there should be evidence of liquid passing by the sight glass .If its R 12 there should be no bubbles just refrigerant ,if there is no liquid at this point your system is empty .It may however still maintain a little gas 40 psi or so nothing to worry about as there will be very little volume .If your system is full you will have around 150 psi to contend with and much more volume and it should be reclaimed properly,if on the other hand you just want to remove the compressor and if you have service valves on your compressor they can be closed (front seated ) and the compressor removed without loosing to much gas ;even on a full system .But you must disconnect the compressor in such a way that leaves the service valves connected to the pipe work not the compressor . Good luck HOPPY