How are doctors able to tell if you have pneumonia?

Crackles on auscultation, fever, difficulty of breathing, presence of green sputum (if it's bacterial, not viral). Those are the usual manifestations but those signs/symptoms are could be seen on other respiratory conditions too. They are only able to confirm the diagnosis by chest x-ray and CBC.

Symptoms of pneumonia are very much like a persistent cold. Cough, congestion, shortness of breath, fever. The diagnosis comes from a chest x-Ray or CT scan that will show that pneumonia is present.

by the sounds of your lungs. i believe the sound is like rustling paper