The only definitive means of accurately diagnosing celiac disease is an intestinal biopsy. This is what they consider the "gold standard" for diagnosis, although nothing in medicine is 100%. Specific blood tests (serological testing) are an important indication of whether or not you have it and for the doctor determines whether he will do a biopsy. Other things play a role; clinical judgement and if patient is sick with symptoms that strongly suggest celiac. If the blood and biopsy tests come back negative and the patient has symptoms then the tests need to be reviewed and maybe even repeated. You may need to get another opinion. Patients in the end know how they "feel". Damage to the intestine , which is shown on a biopsy, is the key component of a celiac disease diagnosis. What is often confused is whether someone has celiac disease, or gluten sensitivity, or gluten intolerance. Either way, complete lifetime removal of gluten from the diet usually results in a "cure" and eventually, a return to good health.

Simple blood tests (CBC) are not 100% since they depend on your diet, but if you have ESR, CRP, or a CMP exam then they are as close to 100% as you will get these days.

From personal accounts of people having a gluten free diet when they are not celiacs, they find it beneficial to them anyways.