The best test to diagnose gastroesophageal reflux disease and quantify acid output is:
"The gold standard for diagnosing and quantifying acid reflux is the 24-hr pH test." -Sabiston
Also, Know
Using 24-hour pH test an overall score is obtained. It is known as the DeMeester score.
Esohagogram
Endoscopy
Manometry
24 hr pH monitoring