How is a prostate exam done?
Sheldon Marks, MD explains in this video.

The reason we do a prostate exam is obviously to feel for any irregularities, any nodules, any areas of firmness, anything that doesn’t make sense. At the same time we’re doing that we’re also looking for any rectal wall abnormalities because that can be an early warning sign for cancer, and we’re also looking for blood which that can be an early warning sign of a colon cancer higher up.