To diagnose pneumonia, the doctor begins with a medical history and physical examination. Bacterial pneumonias often have a sudden onset, creating a sporadic cough with mucus production (often colored or "rusty"), and fever (often with chills), and pain along the chest wall. Infections caused by mycoplasma and viruses typically develop more slowly (often after a few days of flu-like symptoms) and can create a hacky or barky cough, a headache, and pain under the breastbone.

The doctor may order several tests to help diagnose the pneumonia. Chest x-rays are examined for signs of infection in the lungs. Often, different types of pneumonia will have different patterns on the x-ray. Blood and sputum tests may be done to collect a specimen if the doctor needs to determine the organism causing the pneumonia. Furthermore, a biopsy ( taking a lung tissue sample via needle, tube, or surgery) may be done in more severe cases.)