Learn to read labels. The FDA requires that all prepared foods/products sold carry specific information for allergy/celiac sufferers. At the end of a list of ingrediants you will usually see a mention of any of the 8 most common allergens, like wheat, milk or soy. It may also say if the item was processed on equipment that sometimes is used to processes wheat. There are a variety of lists online. Most stores carry some.
Complicated, but... This actually is a very detailed question, much too complicated for this forum. But google gluten free diets and stores in your location for your answers.