Turn the car off, take a big phillips head screwdriver, and find one of the AC line fittings. Unscrew the plastic cap on it and VERY lightly press the pin inside the fitting down with the screwdriver. It releases refrigerant when you do this, so you only want to do this for a split second. If you press too firmly, you can rip the rubber seal in the valve or jam/bend the valve. If no gas is released, it's empty. You need to find out why it's empty (where the leak is) and repair that before recharging it.

If you press the AC button, you should hear a "CLICK" and your idle speed should change. If it doesn't, it could be the AC switch, or the magnetic clutch assembly. But those may not even engage if the AC system is empty, so test the first thing first. Don't forget to check all your fuses, too.