There are many ways to learn what a good koi looks like.
Reading koi books and magazines, attending koi shows and seminars, joining koi clubs and seeing member's collections, reviewing threads on koi forums.
One of the absolute worst ways to learn is by critiquing a picture of a bad example and trying to explain why it isn't a good one.
One of the best ways to learn is to click on this site's banner ads. By surfing these highly reputable sites you can see thousands of examples of koi for sale, which will help to develop your eye. And it's free.