How do you CHECK your trail shoulder is moving downplane?

[Yoda]

I understand that the trail shoulder should move down plane from start down. How do you CHECK with precision that it is truly moving downplane ? I was asked this question today by a friend of mine that just missed getting through first stage tour school and did not have a conclusive answer.

The Feel Check is to turn the Shoulder directly toward the Ball.

The Reality Check is Ball Response.

In between, there is video.

Carpe Diem.