From the internet:
The key distinguishing physical feature that male koi possess develop in late spring as they come into breeding condition. Often mistaken for whitespot, breeding tubercles appear on the head, flanks and tail of mature male broodfish. To the touch, these tubercles make the typically slippery koi feel 'as rough as sand paper' giving them better purchase for when they drive and push against gravid females in an attempt to expel their eggs. In darker koi, white tubercles will be clearly visible but on paler or metallic koi, tubercles may only be evident to the touch.
Females on the other hand, do not develop tubercles nor will they retain the sleek lines of a male as the breeding season approaches. Eggs that develop in the ovaries will swell, leading to an increase in volume to the abdomen. In addition, female koi tend to behave in a less timid manner to a tank full of males, feeding voraciously as soon as the food hits the water. Whether these characteristics are as well defined and identifiable in a pond of mixed koi is debatable, but males are still less likely to be the first to feed of the surface.