The character of pain, location, timing, alleviating and relieving factors, associated symptoms, all provide clues to the possible diagnosis. After looking into the history carefully, focused physical examination and investigations can lead us in the way to find some answers as to the reason for the pain.
Blood tests are most useful in diagnosing acute problems like infections and inflammation where as imaging modalities like Ultrasound, CT and MRI help us look for more structural/ anatomical problems that might explain the symptoms. Endoscopy lets us examine the inner lining of the intestinal tract.
Gastroenteritis, Gallstone disease, Appendicitis, Peptic ulcer disease, Pancreatitis, Hepatitis, Kidney stones, Inflammatory bowel disease, Gastroesophageal reflux, Gastroparesis, Diverticulitis, Urinary tract infection, Ovarian and uterus related problems are among the possibilities can cause abdominal pain. Various cancers (colon, stomach, pancreas, liver, gallbladder, bile duct, kidney) also can cause abdominal pain but with careful testing can be effectively ruled out.
Once in a while, even after doing an extensive workup, we clinicians are unable to point to a clearly identifiable reason for the patients' symptoms, having to put people in baskets like "Functional Dyspepsia" and "Irritable Bowel Syndrome".