How to tell if your AC compressor is bad?

[mookie32]

I've got a mechanic telling me my AC compresser and evaperator/dryer are bad and need to be replaced. I bought the car off Ebay and the AC worked for about 300 miles.
I've purchased a 'parts car' of the same year and modle and I was going to take the compressor and dryer off of it. The engine on the parts car doesn't run and when it did, the former owner said he wasn't getting any cold air off of it. So, without knowing how to tell if the AC compressor is working, I don't know if it's worth the time and effort to take the part off the 'parts car' other than hoping it works and saving about $400 in AC parts.
(It's about another $300 to $400 for labor and freon.)