The Celiac Sleuth: Tips for Spotting Gluten on Food Labels Learn the basics on how to tell if a packaged food product is gluten-free. Identify the sources of gluten. You will never find "gluten" listed as an ingredient when reading a food label. The most common sources of gluten are wheat, barley, and rye. But aside from those, there are various forms of wheat and wheat-based ingredients that you ...