This is a very good question. One place to check is the floor pans. Pound on them from inside the car, not hard, but firmly you will be able to tell if there are any soft or hollow sounding places. Check the trunk, leakage there could go unnoticed for a while and cause some damage. Of course, always look at the frame rails. Surface rust is OK, as long as that is all it is. Check behind the drip moldings above the windows. That is a common place for rust. The real key is, if it looks bad, it probably is. I know that sounds simple, but surface rust is common, and not a problem as long as it is addressed in time. Once it goes past being just on the surface, you have a problem, and you are probably not going to like the solution. ie: replacing body panels, and floor pans.:thumbdown