There are lots of things you *should* check, but I have in the past had mostly good luck throwing in a can of refrigerant and or/sealer (I use 12a HC because 134a is not available off the shelf here, but same idea). Beware of sealers though, I've heard horror stories about some. I only use redtek products and have had no issues, but your mileage may vary.
For $50 or so you can get a cheap set of manifold gauges and they are worth every penny. Check high/low side pressures first to make sure it needs a charge. If not, at a minimum check your low side.
It's usually cheaper and easier to just throw a can in and see what happens than it is to diagnose it properly first. If the top up doesn't work, then you really need the manifold gauges and do some troubleshooting. Out of probably 2 dozen cars I've charged, the only one that wasn't that simple was of course my jeep (compressor was toast).
2004 Imola BMW ZHP
2006 Commander Limited Hemi QD2
"Bought, Not Built"