I can't tell you how much time I've spent trying to teach my parents these simple rules about food labels. Part of the problem is that they don't see very well. My father is diabetic and was told it's okay to drink small servings of 100% juice. He routinely brings home juice that is "100% natural"... which is basically meaningless as sugar is considered natural.
The best way I've found is to eat products that don't have a nutrition label. Fruits, vegetables, meats, and nuts are often sold in bulk. No need to check the label because you know what you're getting.