make sure the throttle will snap back properly.. check to make sure the clutch still has alot of adjustment left, all signals,headlamps, tail lamps are working, see how worn tires are. make sure brake fluid is pretty clear, if its dark, then its never been changed, needs changed.. no big deal,but all these things can be bargaining tools.
and like kenny said look for obvious signs its been laid down.
dont let anyone fool you if there are scraps.. if its just minor nicks... yes it may have been tipped, but scrapes where it looks a if the bike was moving means it was laid down.