The best way to judge the nutritional value of a food is by

Read the food label first. Depending on your weight, age, and gender, you need a certain amount of calories, sugar, protein and other nutrients. Make sure the calorie amount is under 400 calories, unless it is a full meal. (full meal should be around 1000 calories) try to buy healthier foods with less calories, more protein, less sugar, and more fiber! If you're on a diet, you can still eat snacks like FiberOne bars and cookies. Your snacks should be under 400 calories, and you should not have that many snacks!
   Hope this helped!