Using the length of the horns to nose is really good for judging the lower brow tines, if the brows are out to the end of his nose, shoot! I also look for main beams that go past the shoulder. Also that the 4th and 5th aren't bunched up, they should be spaced out indicating better main beam length.
This is good advice, if the 1st and 2nd are out to or gasp, beyond the nose- you've got a bull that you should shoot without thinking about it. Take a look at the bulls that come out of the area you will be hunting by looking at outfitter's web pages or surfing the net. That should give you a good idea what kind of genetics you'll be looking at. Where I hunt, the elk are known for having weak 3rds, sometimes they don't even grow thirds on one side or both.
Also keep in mind that any elk is a trophy, even a a cow for a lot of people.