To check if the compressor is turning on, hit the Auto button on your climate control then move the temperature down until it says Lo. You should hear a drag on the motor, but if you really want to make sure you should remove the panel behind the seats to see if the compressor clutch is engaging. If it's not engaging, then I would start checking relays, etc. It doesn't do much good to check the refrigerant level unless the compressor is turned on anyway.
If you need it, I have a relay chart for the 2000 Boxster on my website at
http://www.theschillings.com/pcars.htm Click on the Boxster site then look under "Where Is It".