Check the sills under each side of the car (beneath the doors). Check the doors, specially down along the bottom - if the drain hole is blocked then water accumulates in the door and starts to rust it. Check around the front and rear window trim 'cos they can rust there a fair bit.
Look under the car to see if there is no diff oil or transmission oil splattered about the place and the floor looks good. Also good to check under the carpets to make sure there's no rust holes in the floor. Check the engine bay to see if there's oil or water leaks, general cleanliness and no leaking brake or clutch fluid.
If you see any bubbles in the paint then push on it 'cos it usually denotes rust underneath. When pushed the bubble breaks away to reveal our old enemy, iron oxide, eating the metal underneath.
If you want to get serious take a small magnet with you and run it over the car, though don't scratch the paint with it. This will help you find any patches of bog repairs.