tb:
I don't have a picture here that I can upload so I'll use this drawing.



In this drawing I have a 7x7x24 timber with two framing squares hanging on it.
One at each end.
Now if you got down low enough so that you could site the top edges of the framing squares the 24" leg horizontally it would show if the timber has any twist in it.
It will show because the square furthest away will not be "in line" with the one closest to you.

I did discuss this with one of my former teachers and he suggested that we use the center point and compare one end to the center first.
And if the end was off plane the end until it matched the center.
Then compare the other end to the center.
And then plane that end until they match.
This would hopefully take half the twist out of the timber on each end.

Hope this helps...