It's a skill acquired from years of looking at cars. The factory paint is usually very smooth. The car is uniform in color. The gloss of the paint is the same from panel to panel. The fit of doors and hoods is correct, straight, and evenly spaced side-to-side.
When a car gets hit, usually the affected panel is replaced, not repaired. The area around it gets primed and repainted at the same time. Sometimes there are non-accident related reasons to repaint a car- parking lot scratches, scrapes, key scratches, "garbage can" scrapes down the side, paint chips in nose and wheelwells. Generally you can tell if the panels have been replaced. Are all teh factory decals and insulation on the inside? under the hood? in the doorjambs? When a quarter panel is replaced, there WILL be obvious welds at the rear door opening and in the hatch or trunk frame. Look at the seams on both sides of the hatch. Look at the inside of both rear doors up the back edge. Both should look the same. It all should be smooth and should match the outside paint. There is NEVER overspray on ANYTHING from the factory. You should not see any paint spray on the door hardware, or on the rubber window seals, or the chrome, or in the corner of the headlights.
IF YOU FIND such items, ask for an explanation. Ask about the car's history, if known. Ask your mechanic if he agrees with whatever you find. If it's untouched and original, great. If it's repainted original bodywork, fine. If it's had a full nose and 2 doors, red flags should go up. Have it inspected much more closely.
Hope this helps. Also see the FAQ, "Buying a 700/900"
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Rob Bareiss, New London, CT 86 244, 87 244, 88 744