How can you distinguish between flu symptoms and a stomach bug?

Flu / influenza: Influenza is primarily a respiratory and not a stomach illness. Flu causes fever, cough, sore throat, sinus pain, muscle aches etc.

Sx's: The symptoms of Stomach Flu (Viral Gastroenteritis) can include: nausea, vomiting, headache, watery bowel movements (not bloody), abdominal cramps and a low fever. According to Mayo Clinic Flu sxs may include “Fever over 100F (38C), Aching muscles, especially in your back, arms and legs, Chills & sweats, Headache, Dry Cough, Fatigue and weakness, Nasal Congestion.”