Earlier we worked on getting the feel both of being on plane and, deliberately, off plane.
Now we should consider how to know what plane we are on. Incidentally this will also help us to stay on it.
There's no right or wrong about it. No plane you must use, no plane you mustn't. It's up to you. The only rule is to know which plane you're on.
To do this, note where your hands are at the top of the backswing and, subsequently, at the finish. That's almost all there is to it.
For an upright swing you might feel that you are swinging from ear to ear, to use an old Scots expression.
For a somewhat less upright swing you might feel that you're swinging from neck to neck. (I know you've only got one neck, but I'm sure you know what I mean.)
For a flat swing feel that it's from shoulder to shoulder.
You will note, I hope, that in all these cases the swing starts from the top. Not from the address. You may have your own way of getting to the top of the backswing. Who cares? The vital thing is to be on plane from the top to the follow-through.
Now, practice doing it wrong as well as doing it right. Recognize the feeling of doing it wrong and mentally discard it. Recognize the feeling of doing it right -- of using a particular plane -- and bank it.
If you do a few swings from ear to shoulder or shoulder to ear I think you will recognize an error you have made many a time before without knowing it.
Now you'll know.
You'll know the plane you want to use. You'll know how to recognize it. You'll know the true feel of it.
And knowledge, you know, is power.