Trained eyes can pick those cars out. Easiest way is to look for orange peel or complete lack there of on a car. EVERY factory paint job has a % of orange peel in the paint. they are mechanically sprayed, and NOT sanded/buffed glassy. Repainted parts from a body shop are always cut and buffed to remove dirt, dust, runs, sags, and excessive orange peel. The repair will look great, and the colors can be spot on, but the finish in the surface will not usually match the rest of the car. OR, they will be lazy and not buff out a repair and you can see more orange peel than the rest of the car. Always look in door jambs for a faint line of hazy clear between the door gap ahead of the seal. a special tape is used, or a taping technique is used to keep excessive overspray out of the jambs when shooting panels. But its not perfect and will leave a slight sign the panel was painted behind on the inside jambs. Hell you could tug a door seal off the pinch weld if you suspected something was up and see if there was a paint line in there. Say a rocker was damaged, you have to pull the gaskets and paint the entire jamb, there is always a tape line somewhere.
I could go on and on, about finding out all these little tell tale signs to a car that was hit getting repaired. I know someone recently got burned on a car this way. Car was a clear title no accidents, but i know for a fact it was creamed in the back and painted all the way up to the front doors. Its not cool when they lie about shit and push a car off on someone that doesnt know exactly what to look for.