You can usually "see" cast unless it's awfully near neutral (no cast). Anything that will project the centerline of the rib over the butt (straitedge and pointer for instance) will allow you to measure. I usually set a small laser level on the rib, get it aligned with centerline of rib to my satisfaction. The beam in most cases clears the top lever and projects onto the top of comb at butt.

jack