Bar codes are verified in many ways. Typically, the distances between each bar is measured, along with the dMin/dMax ratios (the minimum and maximum density). The two charts below show, in graphic form the black bars and white spaces, The bar-code at the right has wider bars and spaces than the one at the left, Both pass and recieve a grade of "A".

   

Also see:
    How do I get the best bar-code label?