Toyota Repair: AC not turning on., ac compressor, freon

Question
My AC Compressor is not turning on.  I replaced the whole thing and went to R12 about three years ago.  I have checked and it is getting power.  I think it may just be the clutch.  I also read that I may be low on R12, and that would make it not turn on, but I don't think this is the case.  Can I just replace the AC Clutch?  or will I need tools and experience that are dealership/AC Shop specific?  Any help is appreciated.

Answer
The first step in diagnosing an a/c problem is to hook up some pressure gauges to read the pressures in the system, the most common cause of compressors not turning on is because there is low freon pressure which is caused by a leak, if the pressures are ok then there may be a problem with the clutch.