The first step you should take when checking your AC system is to run an AC performance test. This test will let you determine if your AC system is up to the task or not. It's a simple test; all you need is a thermometer. They make special thermometers for this that you can get from your local auto-parts store.
Be sure to take your measurement from the center vent. This ensures the most accurate reading. Before you start your test, take a reading of the air temperature around you. This will be your baseline. Then:
Start your vehicle and turn on the AC. Do not put it on recirculate.
Put the fan on the lowest setting and insert your thermometer. In a few moments the temp should bottom out and you can take your reading.
Compare that reading to the temperature reading you took before starting the test. It should be 30º to 40º F cooler than the outside air.
Humidity will greatly affect this reading and the ability of your AC system to cool. The higher the humidity, the harder your AC system needs to work, so be realistic with your assessment. If it's a very humid day, your AC system will have to work pretty hard to do anything at all. If the AC isn't cooling at all, open the hood and see if the compressor is turning. If it's not, then your diagnosis should start there. If it is, your problem lies elsewhere and it's time to break out the pressure gauges.
Here's a video showing how to do the AC performance test.