How do doctors diagnose post nasal drip?

This is usually from the history of the patient. These patients might feel some phlegm or mucus flowing down from the nose to the back of the throat. Some patients feel some mucus or phlegm constantly stuck at the back of the nose or mouth. On Nasoendoscopy, the doctor might see some mucus or pus following from the nasal cavity towards the back of the nose and into the throat.