Originally Posted by Mr.Right

I surmise they use probability based on how the fecal matter exited their anusus.

One can usually sense a creamy bowel movement with plenty of broken chains and hangers-on that require copius wiping. If your first wipe was uncharacteristically slippery and doubles the foul odor in the air, it's safe to say there's much more wiping needed. Best to keep doing so until the tissue feels like sandpaper, and then follow with a shower.

On the other hand, if one practices a high-fibre diet, bowel movements are frequently swift, clean and precise; like a special operations team storming an oil rig taken over by terrorists. In this case, it's often enough to do three conventional wipes along with an insertion wipe, which consists of wrapping your index finger with paper and inserting said finger into the anus and wiping in cirucular motions as if you were trying to get the last tasty morsel of peanut butter from the jar.

Blind people are resilient, and so I have no doubt that the fecal business is handled just as well, if not better, than their sighted brethren.