My stomach has been hurting. How can I tell whether it is acid reflux?

See your doctor. Acid reflux (gerd) typically causes 'heartburn', that gnawing burning sensation in your mid-chest, often worse after eating and/or when laying down. It may be accompanied by difficulty swallowing, regurgitation of food, or even wheezing. It rarely causes abdominal pain.
Trial of medication. A brief answer is to try medication to reduce stomach acid such as Pepcid (famotidine) or Omeprazole would be helpful. The evaluation of abdominal pain as a general rule can be tricky and I would encourage discussing this with your physician to avoid missing a serious medical problem.