There are a variety of tests that doctors may use to diagnose GP. Although the standard test is the gastric emptying scan, it's important to have other tests performed to verify the diagnosis, look for potential causes, and rule out additional conditions.
GP testing can often be difficult as most tests require the ingestion of food or contrast in order to be performed. Many tests require sedation. However, they are all important in getting to the final diagnosis and to figure out the best course of treatment.
Recently, newer tests have become available which are geared towards an accurate diagnosis without the need for ingestion of large amounts of contrast or food. However, these tests are not diagnostic on their own and additional testing may still be necessary.
Most GP tests require a patient to be NPO (nothing by mouth) after midnight the night before the test, or to do some sort of prep. Medications are usually stopped to ensure the greatest accuracy of results. In most cases, testing can be done on an outpatient basis.
As of April 6, 2015, the FDA has approved a new test for diagnosing gastroparesis, the Gastric Emptying Breath Test (GEBT): http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm441370.htm