That is a pretty foolish question.  If you sit there and think about it, wiping our arses, ect, after we goto the bathroom, we are all blind :)\n\nAs to wiping their faces, ect, they only have to 'feel' their faces with their hands to see if they have any 'gunk' or 'mess' on it, and they continue to wipe again, if needed.
How do blind people know when they are done wiping?