Feel the copper lines leaving the compressor. One (usually the smaller) should be pretty warm if not too hot to hold onto very long & the other (usually larger) should be cool if there's refrigerant in it and it's moving heat. If you can measure the compressor amps, check if it's near spec (probably on a tag somewhere). They generally draw more current if they're working under load than if not. If you haven't lost the charge, you may have a valve in the compressor stuck or possibly a blockage somewhere in the refrigerant circuit. You'll probably spend close to $100 to have a tech come out and look - especially if he has to add service taps to check the charge.