Diagnosis of Foodborne Illness

Your doctor may be able to diagnose foodborne illness from a list of what you've recently eaten and results from the proper laboratory tests. Diagnostic tests for foodborne illness should include examination of the feces. A sample of the suspected food, if available, can also be tested for bacteria and their toxins as well as for viruses and parasites.

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