A circular polarizer does not have a fixed starting point or an on/off position. It is always polarizing light; rotating the front ring simply changes which polarization angle is passed.

In practice, you use it by looking through the camera and turning the filter until the scene looks the way you want. As you rotate it, reflections and glare may increase or decrease, and skies or other polarized subjects may change in intensity. Stop at the position that gives the desired effect.

Yes—on a through-the-lens viewfinder system, the effect should be visible while composing, because you are viewing through the filter. The exact strength of the effect depends on the subject and lighting direction, so sometimes the change is subtle and sometimes it is obvious.

So: there’s nothing missing, no calibration mark to line up, and no single “correct” position. Just rotate and judge by the image.