Typically if the refrigerant has leaked out, the compressor will turn with minimal effort. Since it has likely leaked out, look for oily residue around the clutch, slung on the bottom of the hood, fender well, etc. That will indicate the shaft seal is leaking.
The same dirty oil residue on any other component will indicate a leak.
The good thing about the compressor turning easily and smoothly is there hasn't likely been a mechanical failure in the system. Once that happens, it is near impossible to get all the shrapnel out of the system.