For me, the answer has come from a simple mathematical equation.  I take the total number of pages in a book and divide by two.  If my total marginal notes exceeds that number then most likely the book was invaluable to read. 

This does not mean, of course, that I agree with everything in the book.  It does mean the book provoked much fruitful interaction.

The photo above is my copy of the terrific, Flannery O’Connor and the Christ-Haunted South by Ralph Woods.