I just installed a X910BT in my car, and am liking the device so far. I've been checking all of the functionality out before reassembling the car completely (just in case I forgot to hook something up, but I'm not sure how to check if the VSS is properly feeding data to the nav unit. Is there a way to check this?

There is no way to check it. It is speculated that it does nothing anyway. When I had mine, it never seemed to kick in when I drove in a parking garage where I wasn't getting any sat signal.