How does the doctor test for polyps?

The doctor can use four tests to check for polyps:

• Digital rectal exam. The doctor wears gloves and checks your rectum, the last part of the large intestine, to see if it feels normal. This test would find polyps only in the rectum, so the doctor may need to do one of the other tests listed below to find polyps higher up in the intestine.
  
  
• Barium enema. The doctor puts a liquid called barium into your rectum before taking x rays of your large intestine. Barium makes your intestine look white in the pictures. Polyps are dark, so they're easy to see.
  
  
• Sigmoidoscopy. With this test, the doctor can see inside your large intestine. The doctor puts a thin flexible tube into your rectum. The device is called a sigmoidoscope, and it has a light and a tiny video camera in it. The doctor uses the sigmoidoscope to look at the last third of your large intestine.
  
  
• Colonoscopy. This test is like sigmoidoscopy, but the doctor looks at all of the large intestine. It usually requires sedation.
  
  

Colonoscopy or sigmoidoscopy testing

source: http://digestive.niddk.nih.gov/ddiseases/pubs/colonpolyps_ez/index.htm