Hi mingaun,
There's some excellent advice here in this post. For hobbyists without the benefit of years of experience looking at beni, the easiest way to "see" what you're actually looking at is to get accurate facts on the fish - the date it was born, and extremely important, the conditions it's been raised in over the last few months,... and very importantly, the size of the fish at that age,... not necessarily to predict that it will grow large, but to predict how the beni will develop over the next few years.
Ideally to get the very best, and to ensure the Koi hasn't been stunted, isn't a year or two older than it's claimed, or finished prematurely you need to get it either right after harvest, (normally 6 months) or within the 1st 12 months of its life, right after the breeder makes his final selection for the tosai he will grow out to nisai.
For the very best, and best chance at a good future you have to get it "fresh" and you have to continue the excellent care for the next 5 years,... or less if you're finishing a Koi smaller for showing.
Here are some pics and video of tosai Kohakus just after harvest, at 6 months of age - 23 cm to 30 cm. These are at a quality level that wouldn't be sold until their 2nd or 3rd year. Notice the slightly muted look of the beni,... but more importantly the body shapes at this age. These are all Kohakus for the future,... at this point. Pay particular attention to the texture of the beni. Notice how different this is than the eye-grabbing in-your-face red we see on tosai that are for sale. A different purpose altogether.
The handsome mug in the video is my good Koi friend Chuck Shaffer.
Best Wishes,
Brady Brandwood
(streaming video)
http://www.fototime.com/3679A3B703D8C71/conv.wmv
(mac)
http://www.fototime.com/3679A3B703D8C71/orig.mp4