How do you know that the label on a bottle of juice regarding nutrition is correct?
You don't. You'll have to rely on the processor's trustworthiness. You might try comparing the nutrition panel of similar juices to see if something is off. In the United States, FDA does not have the resources to check all labels and test products for label accuracy. If the label seems really wrong, bring it to the attention of the agency with jurisdiction. In the United States, that could be the FDA.