One way I assess my skill as a breeder is whether what I am producing on the ground could have been predicted by the genetics I've employed.
If the foal/horse on the ground is worse than what could have been predicted based on the genetics of the sire and dam then I have made a "destructive" breeding decision.
If the foal/horse on the ground is what could have been predicted based on the genetics of the sire and dam then I have made a "neutral" breeding decision.
If the foal/horse on the ground is better than what could have been predicted based on the genetics of the sire and dam then I have made a "positive" breeding decision and I have added value to the genetics.
I then try to figure out whether that added-value was a happy accident or indeed if I have discovered a potential nick. I do this by repeating the breeding and making new (and ongoing) assessments of the combination.
