This problem first appeared on cars. The rapair shop would look at a car and estimate the repair cost. Then later when they removed a fender/bumper with light damage they found the body work bashed in underneath.
It's possible to make very small glass microbeads these days (they are mixed into resin to make light weight fillers). I was wondering if a version filled with dye could be laminated into a composite structure. So that when "dented" the composite would "bleed" and the dye make any damage visable?
Oh bother - there goes my chances of getting a patent.