“How can you tell the difference between appendicitis and gastroenteritis?”

Doctors who have trained in certain specialties can make that diagnosis.

Appendicitis is simply an infection of the appendix, usually caused by a blockage of the lumen. Symptoms consist of acute right lower quadrant abdominal pain. Gastroenteritis is an infection of the stomach, small intestine or both. This is usually caused by something ingested like a bacteria (salmonella, e.g.). It causes pain, too, but more prominently diarrhea which is how to distinguish it from appendicitis, generally speaking.