On the face of the multimeter, there should be a setting for duty cycle readings you can dial into. On mine, you can measure it be frequency or by percent (you want percent). Now find the X11 diagnostic connector. Put the positive lead on pin 2 (ground) and the negative on pin 3 (data). Yes, it's supposed to be inversed. Turn the key all the way over, but do not start it. Something like 50% or 70% should show up on your multimeter (something to do with federal vs cali emissions?). With the car running, the duty cycle should be within 10% between idle and 2500rpm.

I'm pretty new at this, so this is as much as I understand. More experienced people should fill in the blanks.

Also, a pic if it helps: