One of the first things your GI will probably do is a rectal exam, where he will insert a finger into the rectum. Not sure what all they look for, but will usually do a swab to test for blood. Will also probably order blood tests, but if you have had some done recently, you may see if they will just request the records from the lab or Doc who ordered them. Then, as Nanners said, the colonoscopy. That is the best way for them to be able to see what is going on in there.
Any other GI symptoms? Or just the bloody mucus?