Seeing that the narrow band sensor is a three state device one should be able to use a variety of methods to see the changes occur as long as your sampling of its state change transistion occurs at a reasonable rate (no flutter) to read the transition and tie it to what external influence that caused the change.
Each device DMM, Analog meter, or scope has its issue related to reading fluctuating transition but all can easily show a shift in transition state if that state stays changed.
It would be far simpler to monitor with a analog to digital state reader on a laptop (again with high enough sampling).
Or simply use an Arduino or Raspberry PI and a three state LED indicator.
http://www.enginebasics.com/EFI%20Tuning/Images/AF%20Basics%203.gif
