One way would be to see if there are other symptoms such as fever, vomiting, or diarrhea. The common signs that a baby displays when he/she has gas are:
- Pulling of his/her legs upwards towards the body
- excessive crying after feeds
- forming a fist with his/her hands most of the time
- spits up a lot
- more burping than usual
- flatulence (passing gas)
- abdomen appears bloated and seems tight
If there are other accompanying symptoms, the baby’s abdominal pain may be cause by other conditions. Common conditions include: Colic, Gas pain, Constipation, Reflux, Stomach flu, Other infections, Food allergy, Intestinal blockage, Poisonous substance, Motion sickness.
You can refer to this article for more information on these common causes:
http://www.babycenter.com/0_stomachache-in-babies_11529.bc