A person does not have to have active symptoms of colitis or Crohn's disease to have a colonoscopy, according to WebMD. After age 50, a colonoscopy is part of a preventative measure that guards against colorectal cancer in otherwise asymptomatic people.
However, people who have colitis and Crohn's disease are at higher risk of developing colorectal cancer, says Mayo Clinic. These individuals need to have colonoscopies more frequently. Since the bleeding that is associated with inflammatory bowel disease is also a sign of colorectal cancer, a colonoscopy is needed to rule out malignancies.
A physician uses a battery of tests to diagnose inflammatory bowel diseases, according to Mayo Clinic. These include blood tests, X-rays, MRIs and CT scans.