Hi David
The easiest way is to compare the size with a growth chart. You can find one on my website at http://www.koionline.co.za/growth.htm
However, this is not very reliable, as conditions may restrict or stimulate excessive growth and bloodlines play a major role. Some koi are bred to become Jumbo's or to grow fast. Some varieties also grow much faster.
Bodyshape is often an indication and the intensity of ginrin, skin quality, quality of sumi or hi and various other physical aspects may give you a good indication.
Apparently the scales, fin spine and ear bone have annular rings like trees, and every ring represents one year. For the fist few years it is a good indication, but apparently not on old koi.