Diagnosis involves obtaining a biopsy from the affected area of the gastrointestinal tract. The tissue is then stained and examined under a microscope. Common staining techniques include Hematoxylin and Eosin (H&E) stain, which helps highlight cellular and tissue structures. Special stains like Periodic Acid-Schiff (PAS) and immunohistochemical stains may also be used to identify specific cell types and components.