You can do a reverse lookup on his telephone #.

And as much as I hate to say this, it has to be said: "Test ride? What test ride?!? I've never seen you before in my life!!!". Even a signed handwritten note on a napkin saying "I assume all liability for whatever happens during the test ride" would give some credibility. Right now, it's completely dependent on his morality.

And for future reference, be glad the only thing he hit was a curb