Testing a compressor to make sure it’s gonna be good for moving air is pretty easy, but takes some experience. To see if a junkyard A/C compressor is good or not, you have to turn the center of the clutch and not the pulley. The pulley will spin free unless the bearing is screwed up. What you want is to be sure that the center of the clutch (that is attached directly to the compressor) does spin…with a little resistance. If it won’t turn, the compressor is locked up and is probably junk. If it does turn, you should feel the resistance and even hear a little air moving. This is also how you figure out which line is the inlet and which is the outlet if they are not marked. Just make sure you are turning the compressor in the correct direction: clockwise.