How can you tell if your toddler has a stomach flu?

Vomiting: If he's vomiting and/or has diarrhea most likely he has the stomach flu especially if he's been around other people that have been sick. Usually it runs it's course in 24 hours if he's sick beyond that you should see your pediatrician.

Gastroenteritis: The symptoms of Stomach Flu can include nausea, vomiting, headache, watery bowel movements (not bloody), abdominal cramps and a low fever. If your child has bloody stools or severe diarrhea, Temp of 102 or >, is lethargic, has vomiting for longer than several hrs or hasn’t wet diaper in 6 hrs seek medical attention.