I like the idea of testing relays. You don't even really need to connect any Spikes, though - there are debug lights for every FWD/REV relay output that should be able to tell you whether the sidecar is working as expected. Just twiddle all of the relay outputs to see if things are hooked up. If your setup can drive all of the relay outputs, you should be able to twiddle the PWMs as well (twiddling the relay outputs is MUCH more complicated than twiddling PWMs and GPIO since it requires a number of synchronous signals).
What's the state of the Robot Signal Light?
http://decibel.ni.com/content/docs/DOC-3120
Russ