To be more specific, I use certain checkpoints to determine if a pitcher is using repeatable mechanics from which he can, with an acceptable degree of accuracy, place pitches where he wants with very slight variances in those mechanics. Foot strike placement, glove side position, posture, release point, and pivot foot turnover.
In order to effectively move a ball around the strike zone or cloak your pitch selection, you first have to establish baseline physical positioning that can be repeated at will. You are correct that pitchers will show slight variations pitch-to-pitch. That is by design. Most successful pitchers have developed a baseline delivery which has been muscle memory engrained. From that base point (established repeatable mechanics) they can then use small variances to initiate movement, placement, and velocity changes. So, what we mean by repeatable mechanics is that a pitcher wanting to throw a fastball middle at the knees would show very similar mechanics for every one of those pitches. Certainly, you would see very slight variances in mechanics for a fastball at the neck and a fastball at the knees, for a curve low and outside and a curve inside at the belt.