A good start would be to get a good CARFAX run on it. You could also take the VIN to your local Department of Motorvehicles and get them to tell you the mileage at it's last registration. Every year whenever you have to have your car inspected and you renew the tags the license bureau records the mileage of the vehicle at that time. Your title should have the mileage of the car when it was last titled, unless you are just the second owner. How many miles are on the car?
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1996 Mustang GTS