So how do you know if you have IBS? There's not a definitive test that doctors use to diagnose it; it's a "diagnosis of exclusion," Dr. Rosen says. This means that gastroenterologists typically look to rule out other common GI issues that would cause your bowels to get out of whack, such as celiac disease, ulcerative colitis, and Crohn's disease, she says. A person with IBS usually has a healthy appearing colon, without evidence of any inflammation, even though their bowels can be really erratic and irregular. Your doctor might also run a few blood tests to check for anemia or thyroid issues that could be contributing to your GI symptoms, but for people with IBS, these tests usually come back normal.