I personally wouldn't, unless maybe it was around an empty parking lot at low speed and even then I'd be iffy on it. If they insist, then I'd insist on them signing an agreement to not hold you liable for any damages, and to purchase the bike at the full price if they damage it. In fact, I might insist that they write out the check before you hand them the keys! If they decide they don't like the bike and haven't damaged it, it's always easy to tear up a check.
I've heard to many times of some idiot hopping on for a test ride, only to wreck the bike and say 'Nah, don't think I want it now'.
Also, make sure they have a valid motorcycle license!