There will be two aluminum lines running through your firewall from the engine bay that go to your evaporator in your dash, they'll trace back your compressor and condenser.
Run your car, turn on the AC, make SURE your compressor is engaging FULLY. Feel those lines, one should be freezing cold, the other should be warmer. If they're both warm/hot, you need a charge. If one's cold, the other's warm, and your still not getting cool air from the vents, you have a problem in the blend door.
(and make sure your grabbing the AC lines and not your heater core lines, which are usually black rubber hoses.
I'd just buy some canned Freon at wally world that has a UV leak detector in it, pump some in until you feel cold air (or cold on one of the aforementioned lines) and stop. Go about your business, and check periodically near any fittings for the UV green dye that's in the leak detector Freon.
When you found your leak, fix it.
The old lady's 99 had a slow leak from "SOMEWHERE", and I did exactly this. My leak was at a fitting on one of the condenser lines on the lower driver side. Upon taking the fitting there apart, it was obvious the o-ring had just gotten old/deformed. The PO had pumped TONS of Freon with "sealer" in it, that only crusted at that bad o-ring and made the leak worse. I cleaned the area thoroughly, slapped in a new o-ring, took it to the garage, vac tested etc, and charged it up. FREEZING cold now.
If you're doing this at home without a professional recharge machine, DO NOT forget to put vacuum to the system BEFORE you charge it up. You can rent vac testers at Autozone. This will tell you if you still have another leak BEFORE you waste the 134a on charging it back up. Put Vac on it down to like -30 and let it sit for at least 15 minutes to ensure no leaks.
ALSO, don't forget to inject some oil in the system as well, sure fire way to destroy your compressor if you don't. Even if you don't think you've lost any oil, injecting one OZ into the system won't hurt it, and will give you piece of mind that there's at least something in it.
Also, do NOT replace any O-rings without lubing them up with PAG oil first, that's asking for trouble. and for petes sake, only use HNBR or otherwise AC O-rings.
good luck.