GERD can be diagnosed clinically by a qualified physician based on its symptoms and presentation.
However, there are tests that could be advised to you to confirm the diagnosis such as:
Endoscopy: GERD can be diagnosed by Upper gastro-intestinal endoscopy or oesophago-gastro-duodenoscopy or EGD. In this procedure, a tube containing an optical system for visualization is swallowed. With the tube progressing down the digestive tract, it is possible to view and examine the lining of the esophagus, stomach, and duodenum.
Biopsy: Biopsy does not help to diagnose GERD per se, although, it definitely helps to identify pre-cancerous conditions like Barrett's Esophagus.
Esophageal Acid Test: The amount of time that the esophagus contains acid is determined by a test called a 24-hour Esophageal pH test.
Esophageal Motility Test: This test determines the efficiency of the muscles of the esophagus.
Gastric Emptying Studies: This test helps to know how efficiently the stomach is emptying its contents into the intestine.
Acid Perfusion Test: Very rarely used, this test determines whether the chest pain is of digestive origin.