I have a look at the panel joins and folds also. I've seen plenty of bog work were they didn't really match the original join of the car. The Sills on vans/utes are a good example.

The door frame on my sandman is what gave away to me that the whole side of the thing thing was bog. The lip where the quaterer panel came in and attached to the door frame was too smooth. Drove a screwdriver in 2 inches of bog before I found the original door frame that had been caved in from a nice prang. (don't remember you try this on a car that's no yours tho, hehe)

Or if you're lucky like my wagon, they don't use quite the right shade on repairs, so the bits are the lighter green in the right light are "X marks the spot" for dodgy repairs.

Also remember, that rust seems to be a little bit like an iceberg. That little rust bubble can be a big drama under the paint.