Bronchitis is diagnosed based on symptoms and a physical examination by a physician.
More Info: Generally, tests are not required to diagnose bronchitis. If your physician suspects bronchitis based on your symptoms, he will use a stethoscope to listen to your breathing for any signs of wheezing and to rule out pneumonia. 
Additional Methods to Diagnose Bronchitis
If your physician is unable to make a diagnosis based on symptoms and a physical examination, or if your physician needs to rule out other conditions, tests may be necessary.
Sputum culture. This test can help your physician diagnose whether the cause of your bronchitis is viral or bacterial based on the color of your sputum.
Chest x-ray. A chest x-ray is sometimes ordered to rule out pneumonia.
Pulse Oximetry. During this simple test that only requires a small sensor to be placed on your finger, your physician can measure how well your blood is receiving oxygen.
Spirometry. This test requires that you blow the air from your lungs through a mouthpiece attached to a spirometer. Using this test your physician can measure how much air your lungs can hold.
”Bronchitis – Cedars-Sinai.” Cedars-Sinai – A Non-Profit Hospital in Los Angeles . N.p., n.d. Web. 2 Apr. 2012. <http://www.cedars-sinai.edu/Patients/Health-Conditions/Bronchitis.aspx>.
 “Bronchitis: Tests and diagnosis – MayoClinic.com.” Mayo Clinic. N.p., n.d. Web. 2 Apr. 2012. <http://www.mayoclinic.com/health/bronchitis/DS00031/DSECTION=tests-and-diagnosis>.