Rust tends to hide in places you won't be able to see. Pull up the carpet and inspect the floor for signs of warping/cracking. To do a thorough inspection, you'll need a jack, jackstands, creeper, flashlight, and a "mirror on a stick."
Heamil is correct, you can wrap a magnet in a handkerchief and drag it along the body panels; when it falls off you've found the bondo.
On most older vehicles, you can check the body to frame connection by rocking the car side to side. Rotten body bushings will often make a gruesome squawk when stessed.
