Food Allergen Labelling
"Foods that are gluten-free are now more easily recognised by the information provided on the food labels"
This means that food manufacturers must list gluten on their product labels.  In addition, there is usually a 'may contain' section to the label plus 'cross contamination' warnings that you can check for clarity.

Therefore, when you are out shopping for food, make sure that you thoroughly read the labels on the products you want to buy for any sign of any 'hidden' gluten-containing ingredients.