Do you have a smartphone?
Try checking out lol sumo; its a pretty good app that shows how relatively well you do.
It uses the same stats riot does, so if you get, say, master rank in everything, youre going to get S ranks.
But what other people have said is true; S ranks are obtained when you BEAT the average score. Score is KDA/wards/damage/CS as far as we know, and your score is compared against the average.
If you DO get an S-rank, youre actually making it harder for anyone else to get an S-rank with that champion in that role.