EXACTLY the principle behind the Bud's dyno I have. It uses a multimeter to give you reference numbers.
Basically, you've got to come up with a "baseline" motor -- one that you know works well on the track. All of your other motors will be compared to your baseline motor, and that will tell you whether it runs better or worse on the dyno. (On the track is another thing...) It can also help you figure out how to gear any given motor, as long as you can find that "gearing sweet spot" on your baseline motor.