All packaged foods must have an ingredients list on their label. Get into the habit of looking at the ingredients in foods, particularly if it is the first time you are buying the product.
The ingredients are listed in order of weight; the ingredient that weighs the most is listed first, and the ingredient that weighs the least is listed last. If you find sugar at the top of the list, the food is likely to be high in sugar. Ideally, the first ingredient is a healthful whole food such as a type of nut, legume, fruit, vegetable or wholegrain.
A good rule of thumb is to scan the first three ingredients, as they make up the largest part of what you will be eating. In addition, an ingredients list that is longer than two or three lines or contains words that are difficult to pronounce usually suggests that the product is highly processed.