Anything that looks suspicious can be sampled. There is a shorter instrument, called a sigmoidoscope, that can see the lower part of the large bowel, where most cancers occur. Unfortunately, more and more colon cancers are out of reach of this shorter instrument. The general term for looking inside the body with -scopes is called endoscopy . Often air is also pumped in.
Air is more permeable to x-rays than body tissues.
Neither of these procedures are particularly enjoyable. A colonoscopy is usually done when you are sedated, because it is quite uncomfortable. A barium enema is not as unpleasant, but there is some cramping and discomfort involved. How extensive a search for colon cancer you wish to endure is largely up to you. Varying recommendations dot the landscape of medical opinion.