Sexing koi is pretty similar to goldfish- its very difficult until they reach sexual maturity and even then, it is not easy. Males will develop classic breeding tubercles on their gill plates and pectoral fins when they are in ideal breeding condition. Females generally tend to be a bit rounder or even lopsided. You can also try to vent mature males and females. Behavior can be a great indicator as well. Breeding season is typically in the spring when the pond temperature warms and food becomes abundant unless you live somewhere that is temperate year round.
Heres a decent article on sexing koi and goldfish: