How do I know whether I have ibs?

Check with your doc. Ibs, common, is not inflammatory bowel disease (ibd). At least 3 mos of 3d/mo of pain, fullness, gas, bloat; often alternating constipation/diarrhea, or one more than other. symptoms often relieved by bm. More in women, often starts in teens, worse with stress, food triggers, can vary with menses; can start after GI infection. Usually no bleeding, anemia, wt loss. Can mimic celiac, ibd, inf, other.
Exclusion. Ibs is a diagnosis of exclusion. If there is nothing else found and your colonoscopy and x-rays are normal, then it is likely ibs. Also based on symptoms.