Your doctor will give you a physical exam. They will ask about your symptoms. They may ask if you've recently been in a place where Legionnaires disease is more of a risk, such as a hospital or hot tub, or areas experiencing cases.
You'll have an imaging test of your lungs. This may be a chest X-ray or CT scan. Your urine may be tested for the bacteria. You may need to cough up a sample of mucus from your lungs (phlegm) or be recommended to have a test of your lungs called a bronchoscopy. This can obtain a culture for the bacteria directly from your lungs. These samples are tested in a lab for the bacteria.
If you have Legionnaires disease, this is reported to the local health department. They will work to find the cause so that other people don't get sick.