A 22-year-old is thrown off his motorbike and brought to the emergency room alert. His vital signs appear intact, but he is moaning and complaining of pain in his left neck. He becomes drowsy and examination reveals a dilated left pupil. There is a clear fluid coming out of the left ear. He also has a light nosebleed and a large scalp laceration. His blood pressure is 110/80 mm Hg, respiration rate is 8 breaths per minute, and pulse is 145 beats per minute. The rest of the exam is unremarkable. What test will you perform to determine that the fluid in the ear is not from the brain?