I feel that you really want to "understand" this practise, which one one hand is great and noble, but which is doomed
Sometimes we zone out, sometimes we do not. There is no wrong Zazen.
But if one is zoning out, and there is not a clear vibrancy to sitting,
I dont think you need to tell a difference. If sitting zazen is comfortable, enjoy that, there may come days when it is not I think when
Hmm...might as well stay in bed and Zone out with the wife then. Much more fun than zoning out in a cold conservatory in the dark!
When you zone out, you zone out.
Sit down and let go. Everything.