How is Addison Disease diagnosed?

The gold standard for diagnosis is an 8 am serum cortisol test. According to the Cleveland Clinic, any reading below 15 ug/dL needs further investigation with an ACTH serum test and a cosyntropin stimulation test (also called an ACTH stimulation test.) A flowchart can be found here with a more detailed description: http://www.clevelandclinicmeded.com/medicalpubs/diseasemanagement/endocrinology/diseases-of-the-adrenal-gland/

Usually an injection followed by blood tests

Bloodwork, check potassium levels and adrenal hormone levels. Ct scan of adrenals

There is a test that measures your ACTH. It's quite simple

A GP can diagnose a lack of cortisol, which would indicate Addisons. He should then further investigate the cause of the addisons such as primary or secondary causes, exogenous or endogenous reasons. It may be necessary for a specialist Endocrinologist to investigate this extensively and to assign a course of treatment going forward.

Blood test. Measuring your blood levels of sodium, potassium, cortisol and ACTH gives your doctor an initial indication of whether adrenal insufficiency may be causing your signs and symptoms. A blood test can also measure antibodies associated with autoimmune Addison's disease.
ACTH stimulation test. This test involves measuring the level of cortisol in your blood before and after an injection of synthetic ACTH. ACTH signals your adrenal glands to produce cortisol. If your adrenal glands are damaged, the ACTH stimulation test shows that your output of cortisol in response to synthetic ACTH is limited or nonexistent.
Insulin-induced hypoglycemia test. Occasionally, doctors suggest this test if pituitary disease is a possible cause of adrenal insufficiency (secondary adrenal insufficiency). The test involves checking your blood sugar (blood glucose) and cortisol levels at various intervals after an injection of insulin. In healthy people, glucose levels fall and cortisol levels increase.
Imaging tests. Your doctor may have you undergo a computerized tomography (CT) scan of your abdomen to check the size of your adrenal glands and look for other abnormalities that may give insight to the cause of the adrenal insufficiency. Your doctor may also suggest an MRI scan of your pituitary gland if testing indicates you might have secondary adrenal insufficiency.

Most idealy by the clinical presentation and confirmed by the electrolyte picture of hyperkalaemia with hyponatraemia and hypoglycemia. With a low serum Cortisol and raised ACTH. The causative pathology in confirmed with MRI of the brain and adrenal gland