Some diseases do not have any symptoms. The physician has to take a view based on his physical examination, the symptoms described and some test. Colon cancer is one of those diseases. The symptoms are linked to the colon. Passing blood with stool blood, weight loss, chronic anemia, irregular bowels etc. are some of symptoms. When the physician has these symptoms to consider, he may try medication. If the symptoms persist still, a colonoscopy would be done to look at the colon checking for unusual activity taking place. The physician would then advise you on the treatment