Wheel Wells, Door Jambs, Hatch Area. Pretty much any area that Water can get trapped and sit for long periods of time. Around lights, seals, windshield etc.
Looking usually works you should be able to see the signs of Rust, bubbling paint, peeling paint, etc.
Sometimes if it looks weird feel it.
Don't be afraid to ask if it's got any rust. They should be honest with you.
Like I mentioned in your other question, Mechanic!