Doing a search will provide you with more information about this particular question than you ever wanted to know, but I will summarize the answer to your question for the 200th time.

Go to the low pressure switch on the receiver/dryer, remove the connector and with key on but engine off, short the pins in the connector. If you hear the clutch click, then the system is low on refrigerant. The more proper way to determine this is with gauges. The low pressure switch interrupts the clutch circuit in the event of an empty system to prevent compressor damage.

Good luck,