One of the most common signs that a dog has a stomach ache is the dog eating grass or weeds when it's outside.
The dog eats the grass to help induce vomiting and settle it's stomach.
Some other signs of a dog with a stomach ache is excess gas or loss of appetite or gurgling that is coming from the dogs stomach.
Diarrhea or vomiting is another sign that your dogs stomach is hurting too.