How do physicians diagnose cushing's disease?

Labs and imaging. A lab draw or urine test can tell if your body is making too much cortisol. This is a condition called cushing's syndrome. Cushing's disease is cushing's syndrome caused by a pituitary tumor. Imaging studies (MRI) can locate this tumor.
Several tests needed. Believe it or not, diagnosing cushings disease/syndrome poses one of most challenging and most time consuming medical conditions for a practicing endocrinologist. There is no perfect test the current guidelines recommend several test in order to confirm it. This includes the over night Dexamethasone test, midnight salivary cortisol and 24hr urine cortisol collection (the latter may need to repeated.