Eugene,
A crop that has been pollinated is indicated in several manner.

1. The falling of the petals of the flower. (Canola)
2. A relative change in position the flower head before and after
pollination. (Clover)
3.Change in tissue condition of reproductive parts of the flower -
shriveling of stamens/ stigma. (Fruit)
4. Change due to "tripping of the flower. (Lucerne)
A grower I presume like a beekeeper with his or her bees has "the eye"
and experience, so after looking around leaves the field "happy with the
set" or not depending what has been seen.

Peter