Faecal occult blood test (FOB). It checks for minute amounts of blood in bowel movements - too small to see but that may indicate an incipient problem. Over 55 and you'll get a kit in the post every two years. You 'simply' collect a stool sample over three days, and smear two specimens in a couple of windows (using a cardboard stick - the poo stick that's provided) on a special card, and post it off to the lab. Easy really!
After my last one they wrote back saying my result was unclear and to do another. The second one was positive so I had to have a colonoscopy. That's the camera on a long flexible probe passed up and round the large intestine looking for abnormalities. They found a couple of small polyps which they snipped off as they passed, and minor internal haemorrhoids - the source of the blood traces found in the test. No nasty lesions though -
Before the colonoscopy you have to have the mother of all laxatives. Either Picolax or Moviprep. The experience is well documented by the author of the forum post http://singletrackworld.com/2009/02/the-picolax-thread-returns/ here.