What is tested when you are asked to give a stool sample? is there something that alerts them to any types of cancer in the test?

See below: If the stool sample is to check for blood, then that is all that is tested for. Presence of blood, even microscopic, can be a suggestion for possible cancer. It can also be due to hemorrhoids, anal fissures, constipation, or intestinal infections. If colon cancer, the type/stage cannot be determined from the stool sample. That is why a colonoscopy is then done if the stool sample is + for blood.

Blood in Stool: The most common test would be for blood in the stool. This can be a sign of colon polyps or cancer. It is also possible to test for certain types of intestinal infections in a stool sample. Stool blood testing is commonly done on people who have no symptoms. Testing for infection is usually part of the evaluation of symptoms (such as diarrhea).