The Universal Bar Code, introduced in 1973, is on every product that you buy. It allows merchants to tell exactly the inventory on hand and significantly speed up the check out process. The numbers on the bar code contain a formula that checks its validity. With the amount of counterfeit and fake products in the marketplace, it's important to know how to spot a fake bar code.
Add the first number in the bar code to the third, fifth and seventh (and ninth and 11th if its a 12 digit code).
Multiply the sum in step one by three.
Add the second number in the bar code to the fourth, and sixth (and eighth and 10th if its a 12 digit code).
Add the final values from steps two and three.
Subtract the number found in step four from the nearest multiple of 10. The number you had or subtract should match the last number of the bar code, the check digit. For example your final value from step four is 127. Add three to get to 130 (a multiple of 10). The check digit is three.