Hi Neli,
A Judge taught me at one of our Koi shows that when he chooses a Showa he uses the following criteria:
1- Body Shape
2- Looks at the koi as if choosing a kohaku (strong Hi,and nice kohaku pattern)
3- White on the koi (must be good)
4 Lastly Sumi
5 Head should have 3 colours
Don't know how accurate this method is but hope it helps you a bit