A scope certainly would be able to tell you if the Jag is trying to drive the motors correctly.
I think it would be helpful to have a hand-held PWM signal generator to be able to test Jags directly. Something like this:
http://www.rc-cam.com/servotst.htm
or this:
http://www.sentex.ca/~mec1995/gadgets/servo3.htm
Also, any hobby store should have something similar available. Google for "servo tester".