AC compressor/clutch makes a hell of a squeal when it first kicks in. How can I tell if I need a new clutch, compressor, or both?

Once you have ruled out that the belt is too loose and or worn--You can disconnect the connector and put 12v right on it with the engine off and the belt loose. see if you can make it slip by hand. If so, clean it up with brake parts cleaner and try again. Still slips- replace the clutch. Don't need to remove the compressor to do this either.

-Chuck-

Do I try to make the face of the clutch (with the 3 metal tabs for the pulser) slip, or the belt pulleys?

Belt was not loose or worn. I loosened belts and turned pulley/clutch by hand. Turned smoothly. I put 12V on the clutch connector. Clutch engaged the pulley and both turned by hand. I could not get the clutch to slip in relation to the pulley when I turned them by hand. Both were locked together well with 12V. Does this mean that the compressor is bad? Could the squeal be from low oil in the compressor? Could the squeal be from the idler pulley, or would that squeal all the time?