The diagnosis test (for me anyway) was a barium breakfast, so fasting then the worst flavoured ready-brek you've ever had followed by X-rays. Lots and lots of X-rays. Ten consecutive one minute pictures frontwards then backwards, followed by a ten minute X-ray (try standing as still as possible for ten minutes, yuck). Repeat every half an hour until the barium starts to leave the stomach. It took me all morning, which gave me a positive diagnosis.
You can see why doctors don't want to order the test though - it ties up one tech and one X-ray machine for at least half a day.