A doctor can diagnose the fissure after a physical examination of the anal area. Rectal Exam: It involves inserting a finger or a small instrument into the rectum, which is painful. A specialist will use anesthesia on the area before the rectal examination. Sigmoidoscopy or colonoscopy: A flexible viewing tube is used to inspect the insides of the anus and rectum. This diagnostic test may be ordered if the doctor wants to rule out more serious anal diseases.