The Vehicle Identification Number (VIN) is both a 17-character code unique to an individual car and an easy way to gain some knowledge of the car without even having to see it. A VIN can tell you the car's manufacturer, model, engine type, year of manufacture, where it was made and even what number it was off the line at the assembly plant. The VIN of an Audi A4 will always have at least 5 characters in common at all times.
Find the VIN. The VIN is on the upper left portion of the dash, easily viewable through the windshield.
Identify the first three characters. All Audi VINs should begin "WAU," which describe the vehicle as being German (W) and manufactured by Audi (AU).
Identify the seventh and eighth characters. These characters should read "8D." The only Audi cars with "8D" in the seventh and eighth positions of the VIN are Audi A4s.