A lot of people have asked me how I was diagnosed. The first test for Celiac is a simple blood test, and then after that, you may be sent for an endoscopy. Personally, I was only sent for a blood test. My doctor didn't suggest an endoscopy, and by the time I had done enough research to ask for one, I'd been eating gluten-free for awhile. Ah well.
If you think you or a loved one may have Celiac Disease, just ask your regular doctor to send you for a blood test. However, you have to be eating a regular amount of gluten at the time of the test.
For any other questions about diagnosis, check out this FAQ from Celiac.ca.