Check along the bottom edge of the car for rust; you may need to squat down low to see it.

Pay special attention to the area around the jack holes; in northern climates salt collects in and around these creating rapid and severe rust.

Check for rust up under the fender wells especially on the side where the battery resides up front. Battery fumes will speed up rusting there.

Also check for rust where the fenders meet the doors. Water drains are located there. If they clog water can collect creating rust problems.

Check for rust at the bottom of the inside of each door, you may need to push the trim out of the way