When looking at a used car, I usually begin my inspection from the rear end. Always start by checking the seams (the gaps in between each metal piece). Very rarely would a car in its original form have nonmatching gaps on one side compared to the other.
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One common thing that bad body shops do when repainting a rear bumper is that they leave over paint on the exhaust tips, canisters and other areas under the car around the bumper area. That is an easy way to check for repaint of the rear end.
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After you have checked the seams on the back of the vehicle and the under body of the car around the rear bumper area, pop the trunk. Every visual piece of the car from the mid 90's and newer (depending on the manufacturer) should all have VIN stickers (Vehicle Identification Number) on them. The VIN sticker represents the original pieces of the vehicle in which it was built from the factory. VIN stickers cannot be duplicated and the numbers should be matching with the VIN on the driver's side dashboard. I usually memorize the last 4 digits of the VIN on the driver's side dashboard so I can just look at the last 4 digits of the VIN stickers and immediately identify that they are the original ones. Here is how a VIN sticker looks like.
(pictures will be available soon) VIN sticker
(pictures will be available soon) VIN label on driver's side dashboard
Look at the hinges on the trunk and the screws on the edges. Sometimes when the trunk area gets painted, the hinges and the screws will be smothered in paint leaving no gaps between the screw and the metal piece that it's screwed onto. Lift the piece that covers the spare and look at the paint on the metal around the tire and the shape of it. A vehicle with a heavy rear end collision will have a noticeable difference on the paint and a deformed shape on the spare tire well. Sometimes there could rust as well.
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Head towards the side of the car. Check the seams and the VIN sticker on the door. Check to see if the paint color on the door matches the paint color on the body of the car. Open and close the door a few times. Try to notice if the door has any sort of strange resistance when closing the door. Often times when a door has been replaced, the door does not close smoothly. Check the rubber around the window area. If the rubber around the window has a strange bend like it was pushed in by a bar, the vehicle could have been previously stolen and stripped.
(pictures will be available soon) Hood VIN Sticker
(pictures will be available soon) Side Fenders VIN Sticker
(pictures will be available soon) Front Bumper VIN Sticker
Go to the driver's or the passenger's side of the car that has not been inspected and do the same as you did on the opposite side.