Diagnosis of Viral Gastroenteritis

If you think you have viral gastroenteritis, you may want to see your doctor, although many people don't bother. Doctors generally diagnose viral gastroenteritis based on the symptoms and a physical examination. Your doctor may ask for a stool sample to test for rotavirus or to rule out bacteria or parasites as the cause of your symptoms. No routine tests are currently available for the other types of viruses.

source: http://digestive.niddk.nih.gov/ddiseases/pubs/viralgastroenteritis/index.htm