What Is an instrument used for visual examination of the stomach?

The instrument is called, in general, an Endoscope. There are a variety of versions for specific examinations; for example the Gastroscope for examining the esophagus and stomach; the Proctosigmoidoscope and Colonoscope for examining the colon; the Cystoscope for examining the urinary tract; the Arthroscope for examining joints; and several other variations. My favorite exam name (I love its tongue-twisting length) is "Endoscope-assisted Retrograde Cholangiopancreatography".