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It depends on where the damage may have been. If it is on the front fender, look for any lines where the body may have been cut (since it is all one piece and they rarely replace it all unless needed). If there was damage you can usually tell if you look real close. Usually you can see a line where the cut was made and welded back together, unless the body shop did a really good job. Any detachable parts would just be replaced (i.e. rear bumper/doors/etc.) Look for areas where the paint is a slightly different color or shade. Look for areas that may not have a smooth finish (wavy or bumpy is a possible sign of bondo). Also look at the front and rear bumber to see if they are stock or aftermarket, most bumbers are replica (since most are discontinued)type bumpers and may have markings on the inside and may not fit perfect. Other than that I cant think of anything else. Again, all those things can be covered up by a skilled professional, so no guarantee's.
look under the car.
Look at the backside of the panels
Look at the glue joints between panels, again from the backside.
Factory panels are smooth and dark grey.
seams in the wrong place are also a giveaway
Panel adhesive can also be a giveaway even if good pressmolded or NOS parts were used.