Climb inside. If you detect a musty or moldy odor, suspect a wet interior. (If the interior reeks of a heavy masking scent, suspect the same.) Seats should be free of rips and tears, and should not sag when you sit down. All windows should move freely up and down, and all doors should open, close, and lock properly (don't forget the tailgate). With your helper outside, turn on the lights (low and high beam), turn signals, back up lights, and so on.
Don't forget to inspect accessory equipment such as caps, roof racks, winches, and brush guards. Check each thoroughly.