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. I casually whip out a simple diagnostic tool that consists of a metal pin pressurized by a small, taut spring with a locking device. Push the pin into the frame until the tool locks. If the frame gives way, it's rusty. And rust is forever.