A product label really does not all you to distinguish how much added sugar might be in a product. However, you can compare the sugar content listed for a unsweetened product to the amount listed for a sweetened product of the same kind. An example would be sweetened grape juice vs. unsweetened variety. The difference between the two kinds would be the added sugar--as long as you are comparing equal serving sizes. Also, remember that there are about 4 grams of sugar per teaspoon.
PositiveTip: If you don't want to compare labels just make sure you purchase the unsweetened product!