I am not sure which part of the small intestine can be tested for celiac. The duodenum, jejunum and ileum are all somewhat different. I have only heard of it done via EGD.
But, the first diagnostic step is not a biopsy, but an antibody blood test. If you suspect celiac, I suggest you ask for that first.
https://celiac.org/celiac-dise...sing-celiac-disease/
Jan