The gastric empty test is the gold standard to diagnose gastroparesis. It is a very easy test...just eat some scrambled eggs laced with contrast (you can't taste it), then lay down flat for 2-3 hours and watch TV, listen to music, or nap...just can't move. Images are taken over the 2-3 hours to track the food/contrast and determine the empty time.

Sorry for the short reply. I'll try to elaborate more later.