I am in the same situation as you--I have drawn a chance of a lifetime, and have the most realistic chance for a 350+ bull that I will every have. One way that I am trying to learn to judge them is just by experience--looking at photos to be able to compare the differences.
One place I found on the web is
Bull elk photos A big bull has a squarer rack as opposed to a more narrow U shaped rack. Then I think tine length comes into play.
