There are no specific tests for gastroenteritis, thus gastroenteritis is most often diagnosed by the symptoms it produces, mainly diarrhea. Because gastroenteritis is usually a self-limited disease, the large majority of people are never seen or diagnosed by a doctor. However, during outbreaks like those seen on cruise ships, viral and bacterial cultures or PCR and other immunologically-based tests can eventually identify the causative pathogen. By the time this identification occurs, most of the individuals with gastroenteritis have begun to recover.

When gastroenteritis symptoms become severe, most public health officials and health-care professionals run such tests to identify the causative agent of a specific disease, based on all of the history of the patients, physical exams, and symptoms. In addition, patients with similar histories of recent food or drink they had in common with others often helps to discover the source of the disease (for example, people who got diarrhea had salads from the same food source).