Today's the day! I just want to get it over with.
Lots of people are asking what they're looking for. Here's what I understand (anyone feel free to correct me if there's anything inaccurate):
Basically, there are two things. The first is they want to look at the condition of the small intestine. If it's Celiac, they might be able to physically see scalloping on the lining of the intestine (the top picture shows healthy anatomy, and the bottom is what it looks like with Celiac).
Sometimes that damage isn't visible though. So they also plan to take a biopsy of the lining, which contains villi, finger-like projections which absorb nutrients from our food (shown on the right photos). If the villi look short and worn down, that means it's Celiac. Celiac results in damage of the villi, so this is a definitive diagnosis.
I'm actually hoping for the diagnosis because otherwise I think it's going to be a long, difficult road to figure out what's wrong. I've been feeling so much better just from reducing gluten, I'll be really surprised. But it might take a couple weeks to get the biopsy results. This process drives me crazy!