Is your air conditioning working as it should? Does it seem to work fine at times yet not at all other times?
Assuming your system is holding proper charge, your problem could be as simple as this little switch. It mounts right on the accumulator, thats the large cylinder looking component on the firewall near the passenger side.
How do I diagnose this to be sure its at fault?
First, we are going to assume that the system has the proper level of charge in it. If operation is intermittant, read on.
If your compressor will not engage, give the switch a slight tap with a screwdriver or similer item. If the compressor suddenly starts working, your switch is likely faulty. Another simple test is to bend up a paper clip in a 'U' and jump the two terminals on the connector while un plugged. If the compressor then engages, you can be sure the switch is at fault. Replacement is just as easy as it looks. Disconnect the wiring connector, and unscrew. System discharge is not required with this repair. You can expect to hear just a slight hiss as the sensor is removed from the accumulator. That is just a small amount of pressure relief.