You can be unaware that you have bad breath (halitosis), because it is difficult to smell your own breath. Occasionally, someone will tell you that there is a problem with your breath. A dentist will smell your breath and determine whether further tests are warranted. They may take a sample of bacteria from your tongue, by scraping with an instrument.
A medical doctor can also diagnose bad breath (halitosis). If the condition is chronic, a doctor may order other tests to determine an underlying cause. Doctors and dentists will also take a medical and dental history and ask you questions about your lifestyle, which could affect your breath.