Medical exam
The symptoms of colorectal cancer are very nonspecific, so common to many other pathologies. This is why your doctor, who has been professionally trained to look for evidence, needs to do a thorough physical examination.
The doctor will carefully palpate your abdomen, looking for masses or enlarged organs that suggest an anomaly.
You might also need to do a rectal exam to search for any abnormalities.
In case of doubt, and taking into account your family history, the doctor might request an imaging test.