When Doctors think you might have Celiac disease, or, if you have had a blood test that said are suspected of having Celiac disease, the next step is to have an endoscopy.
Before this though, they ask you to eat gluten for six straight weeks, so they can then look at the effect the gluten has on the tiny bumps (villi) that line your small intestine.
The villi help your body take in nutrients from food into your bloodstream. People with Celiac Disease are often nutrient deficient, so this can be serious.
This is my personal experience.