Most of the advice above is correct. I'd start by charging it with some R-12, which should only cost ~$50 for a can or two. See how long it takes to leak. If the system works at all now (the compressor turns, right?), you do NOT have a bad leak. A recharge might make it work fine for the rest of the summer. If it's weak again next spring, then worry about fixing it. If it only lasts a few weeks this year, then yeah you need to use a sniffer and/or UV dye to diagnose.
The compressor should be FINE as long as it is quiet. When they fail (or are failing), you'll probably know it. I wouldn't replace it outright unless you're made of money. Do not convert to R-134a. Stick with R-12. Yes it's still available. No it's not that expensive. The condenser failure is not terribly common but not rare either, let's hope you don't need to go there.
Oh yeah - make sure all the vacuum pods that actuate the ACC duct flaps work. Pull the glovebox, test all 7 lines with a MityVac. That alone can have a pretty significant effect on A/C operation. It's a good 8-10 hour job to pull the dash and replace ALL the pods (don't just change the bad ones) if it's your first time. Lots of photos on my website for this.
Another thing- make sure the condenser & radiator fins are very clean - poor airflow means poor cooling. And make sure the auxiliary fan works. If you discharge the system, replace the "green" pressure switch with a "red" switch, which triggers at a lower pressure, runs the fan more often, and keeps you cooler inside.
Finally, consider installing window tint that blocks UV and infrared thermal energy. There are clear films available of you don't want dark windows, and the clear stuff works great on the windshield too. I plan on installing the V-Kool or Huber Optik stuff this summer on most of my cars. More details on that are here:
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