The right way to inspect a system, as I understand it, is to extract the old refrigerant under vacuum (it is stored, not vented off), add a dye and fill the system back up. While operating the a/c, the tech looks for dye leaking out as well as having a tester/sniffer device that can detect smaller or harder to see areas of lines/connections, etc. If it leaks it should be repaired. If the system has to be opened for repair or for any other reason, there are always parts that should be replaced. A/C repair is not cheap, but consider this: reputable shops would love to extract your R-12, assuming there's any in there, and they should buy it off you if you're switching the system to R-134a. This can work to advantage, obviously, if you can find a shop that does it. I'm afraid too much of it still gets vented to the atmosphere and either doesn't get recycled or the customer is unaware of its value and doesn't get any credit for it.
R-134a is available at auto parts stores, along with cheap hose and gauge sets. Their effectiveness is limited, however, as a single indicator is only going to tell you the low side pressure, and not give you any information (or little info.) about what's happening on the high pressure side.
Improper use or careless handling of refrigerant can be harmful or fatal, folks. Accidental discharge of high pressure refrigerants can, for example, freeze your eyes, causing irreversible damage (blindness). Freon/R-12 (I have read) burns with an invisible flame, which does not sound fun. In the day, I could buy a can of freon for well under $1 and add one to my car every day, if I had to - - if it leaked, no big deal. These days, I'd say to have it professionally worked on so as to not throw away a lot of money, or damage something valuable because you don't have the tools, equipment and training to do the job right.
I really like this site "How Stuff Works." They didn't have anything I could find on automotive a/c, but this article covers the basics:
A/C Basics Please, if anyone notices incorrect info in my post, feel free to fire away; I deal in floorcovering materials for a living...
John B.
