How is esophagitis typically diagnosed?

Endoscopy. Though there are features seen in a ct scan and barium swallow which are suggestive of esophagitis, the only (and best way) to definitively make the diagnosis is with an endoscopy.
History and tests. Proper history taking can lead the doctor to make the diagnosis. A diagnostic test EGD( Esophagogastroduodenoscopy) can also be done to make the diagnosis.