--can be done at the same time as a standard endoscopy--the tube just has to be inserted farther down, past the duodenum (samples can be taken there) and into the small intestine proper (where more samples can be taken). What is looked for is evidence of lymphocytic infiltrate and villious atrophy.

It's best to have this done with an experienced gastroenterologist who knows about celiac. though. Multiple samples from different areas should be taken, as damage from celiac is often patchy and hard to find, and it's easy to miss damage spots and only get "normal" samples--one of the reasons, traditionally, for celiac underdiagnosis.