A digital examination of the rectum is part of the routine examination of the rectum by every doctor and should be able to palpate the majority of rectal cancers.
A faecal occult blood test should alert the doctor to a bleeding lesion (not necessarily a tumour) in the bowel.
However, the most important test is the proctoscopy and colonoscopy where the doctor directly visualises the entire rectum and colon and is able to biopsy (take for analysis) any parts of the lining wall (the site of origin of all cancers) that look suspicious for cancer.
An indirect alternative to examine the large bowel is the barium enema where the colon is filled with radio-opaque barium solution such that a negative image is projected on x-ray film and any irregularities of the colonic wall are noted.
The biopsy for microscopic (histological) analysis is taken at the time of the colonoscopy and once the diagnosis is established, then a CT scan of the abdomen, pelvis and chest is performed to determine if the disease has spread.