How can doctors tell if you have viral or bacterial bronchitis?

Evaluation...: The symptoms of bacterial bronchitis are usually a bit more severe as in higher fever, more severe cough, more body aches, etc but there is overlap. When in doubt, a sputum culture can be useful. Also, viral bronchitis will usually begin resolving in a few days without treatment so if you are feeling better in a few days w/o antibiotics, chances are it is viral and antibiotics will be ineffective.

Cultures&Microscope: Sputun is examined under a microscope and bacteria can be seen. Cultures are allowed to grow few days and pathogens grow and can be identified. Blood tests can show antibodies to various viruses. Clinical findings can help differentiate in some cases.