I second Red_34, a colonoscopy will allow the doctor to take biopsies and to actually see your colon. Thought that was the only way to get a 100% diagnosis. Demand one, nobody will take care of your health as well as you do. Don't worry about
having one, it really isn't too bad, the fear is worse than the procedure itself.
My fear when I first went to see my GI was cancer too, but he told me that one symptom of colon cancer is unexplained weight loss, which I did not have. If your doctor suspected cancer, I am sure he/she would have you in for a colonoscopy immediately so I would not worry too much.