Do a VIN history, check the VIN # on the actual car to be sure there all there and matching (firewall).

Some manufacturers stamp VIN #'s on major panels i.e. Honda/Acura, if one is missing then likley the panel was replaced.

Check for overspray on moldings and the underside of the car. If you are able to get far enough underneath look at the bumper absorbers (you can tell if they ever compressed). Check for overspray around the bumpers and on fender liners etc..

If there is a same model vehicle on the same lot then you can compare the two right next to each other.

Stick you hand under the dash and find "ledges" or edges, looking for sand or evidence of a flood vehicle.

Look for extensive rust underneath car if a northern car.

Look at the A/C and Radiator fan to see if it may have ever hit against anything i.e. been pushed into something in an accident and never replaced.

Those are just a few thoughts.. If buying from an individual I would do a VIN history to be sure it adds up and check all the VIN#'s to be sure your not getting some chopped up stolen car that was re-titled using a different VIN #.