Agree w/ Dr. Nguyen. Diagnosing Diabetes insipidus (DI) is based on symptoms, medical history, and lab tests. Differentiating kidney-level DI and central (brain/pituitary) DI can be done by measuring vasopressin levels (very difficult, only a few labs offer this test) or observing how a person responds to an injection of vasopressin. Central DI = low levels of vasopressin and a strong response to a test dose.
Blood test. Blood tests, dilute urine , frequent urine.