Theoretically,the best way to is to have the freon recovered by a refridgeration tech with the proper recovery equipment, the other alternative is to put a clamp (vampire) type valve on the low side as near as possible to the compressor itself and "check" for any remaining F-12 in the system...of course
these valves are prone to leakage and you wouldn't want to intentionally discharge bad old F-12 into the "ozone layer" now would you?
'> then have the compressor cut loose from the rest of the system to prep for shipping.