If it wont' hold somewhere, it's because what looks like steel is probably bonded. Then I walk to the front or tail end on one side of the car and knowingly gaze down its length. I'm checking to see if the panels line up with the door. The hood is next. If it's not, I ask if the car has been a participant in any accidents because body repair is always a little off from factory work.
Bubbled paint is another sure giveaway.
Now I open the doors and examine the rubber seals. Cracked or missing. Rubber means leakage. Leakage means rust inside. The frame test.