A 58-year-old man is referred to your office after evaluation in the emergency room for abdominal pain. The patient was diagnosed with gastritis, but a CT scan with contrast performed during the work-up of his pain revealed a 2-cm adrenal mass. The patient has no history of malignancy and denies erectile dysfunction (ED). Physical examination reveals a BP of 122/78 with no gynecomastia or evidence of Cushing syndrome. His serum potassium is normal. What is the next step in determining whether this patient's adrenal mass should be resected?