a/c compressor question

I took my car to a guy my dad knows to have my a/c looked at. He told me that I needed a new compressor for sure and he couldnt tell me what else until he installed a new compressor. So i bought a compressor off of someone on g2ic and today I took out my old one. I was comparing the two and the seem exactly the same. both of them spin and make the same noise when spinning. I guess mine could still be broken but how would he be able to tell? could someone please Help me out with this.
thanks Drew

I thought that the only way to tell if a compressor wasnt working was if it was siezed and didnt spin at all. Nobody can help?

download chapter 22 from my sig. from a quick scan it looks like theres a long list of checks before determining the compressor is bad. somehow i really doubt the mechanic went through all of em.