There is a blood test (I think you have to have been having gluten for the test to be accurate). And the gold standard is a biopsy. There's also a genetic test. We don't have it (but both DD2 and I have had the blood test). We are both gluten intolerant, though not celiac. A friend of ours has two boys who are type 1 diabetic and also celiac (no symptoms, they had the blood test, tested positive, then had the biopsy; diabetes and celiac are both auto-immune diseases that are carried on the same gene apparently). I also have a neighbor who's a celiac, and her two kids are gluten intolerant, but not celiac. There are many people that find the "diagnosis" (and the testing) irrelevant if there is an obvious problem with gluten, because either way you have to remove it completely from their diet.
Kathy, mother of 3, wife of 1. My new recipe blog: www.kathysrecipebox.wordpress.com (no longer searchable by allergen, but at least it doesn't have a virus!)