Learn about the final digit. The final digit is called the 'check digit,' and is automatically determined by putting the previous 11 digits through a mathematical formula. The purpose of this is to catch printing errors. While fake UPC barcodes do exist, usually created by companies who don't understand that they need to apply for one, it would be easy to include the correct check digit, so this is likely not a reliable method of finding fakes. (For that purpose, look it up in the official database instead.) If you're curious or enjoy doing math for fun, you can enter your barcode into a GTIN-12 check digit calculator, or follow the checking formula yourself:

• Add all the digits in the odd positions together (the 1st, 3rd, 5th, 7th, 9th, and 11th digits).
• Multiply the result by 3.
• Add to this the sum of the even-positioned digits (the 2nd, 4th, 6th, 8th, and 10th) - do not include the check digit itself.
• 'Chop off' everything except the final digit of your answer, the number in the ones place.
• If that number is 0, that is the check digit.
• If that number is any other digit, subtract it from 10, and the result is the check digit. For example, if the previous step resulted in an answer of 8, you would calculate 10-8=2. This answer should be the same as the final 12th digit of the barcode.