Can a doctor tell if you have colon cancer by taking test?

Colonoscopy: The best test to find colon cancer is colonoscopy, which is a fiber optic camera evaluation of the colon. Blood tests such as CEA levels are not used to screen (i.e. Check) for colon cancer in asymptomatic patients.

A biopsy.: A colonoscopy is the best way to screen for colon cancer. If there is anything suspicious seen on the colonopscopy, the doctor can take an immediate biopsy. The biopsy is sent to a pathologist who reviews the specimen and gives the diagnosis of colon cancer if it's there.

Depends on type: Guaiac for blood in the stool can be a sign of colon cancer. Colonoscopy is a screening study, where a polyp, tumor, or mass can be biopsied and a diagnosis given by a pathologist for colon cancer. Imaging studies may suggest a tumor also. There are many ways of screening for colon cancer (usually started after 50 years), depending on age and symptoms.