Your doctor will diagnose bronchitis based on a physical exam and your description of the symptoms you've been experiencing. During the physical exam, your doctor will probably listen to your breathing with a stethoscope. Blood tests and an examination of the mucus you produce when you cough might be necessary. Your doctor might also conduct a what is known as a pulmonary function test, where you blow into a spirometer, which is a device that measures how much air you breathe in and out. In some cases, the doctor might decide an X-ray of the chest is necessary before a diagnosis can be definitive.