Usually, colon cancer doesn't have any symptoms. However, in the later stages, symptoms may include thin stools, cramping, unexplained weight loss, and bloody stools.
Reference:
coloncancer.about.com/od/cancerpr…
Another reference: ccalliance.org/about/disease/…
The following symptoms may indicate colorectal cancer. A doctor should be consulted for the appropriate diagnostic tests.
* a change in bowel habits
* diarrhea or constipation
* feeling that the bowel does not empty completely
* vomiting
* blood in the stool
* abdominal discomfort (gas, bloating, cramps)
* weight loss for no known reasons
* constant tiredness
* unexplained anemia (symptoms are paleness of the skin)

For colon cancer prevention tips, go to
coloncancer.about.com/od/cancerpr…
which gives smart, general health improvement tips as follows:
1. go to a doctor if you have any colon cancer symptoms (See above).
2. If you're 50 or older, schedule a colon cancer screening.
3. Eat a balanced diet.
Diets high in fat and cholesterol (especially from animal sources) have been linked to increased colon cancer risk. High-fiber diets, however, have shown a protective effect. (Learn more about this colon cancer prevention tip.)
4. Maintain a healthy weight.
5. Maintain an active lifestyle.
6. consider genetic counseling.
7. Learn your family medical history.
8. Talk to a doctor about your personal medical history.
9. Don't smoke.
10. Reduce radiation exposure.