Most areas in CO, you're kind of living a lie if you're trying to differentiate a 280 bull from a 320 bull from a 360 bull. If it's a legal bull it's a keeper, and if it's a 6 by 6 it's a trophy.

Bugle magazine is a great source for practicing sizing up bulls, lots of photos. I think the one skill any aspiring trophy hunter should acquire is the ability to tell instantly if it's a 6 pointer or bigger. You do this without counting 1-2-3-4-5-yup 6 points, oh boy oh boy! Look at enough photos until you see what people mean by the "sword point", that's the G-4 point. It is the dominant point on 99% of elk racks out there from a 4-point wimp to 7-point monster. Then, if the bull has a fork behind that sword point you know it's a 6-pointer. No fork, it's a 5-pointer, and almost certainly not a trophy.

Beyond that, the more steps you have in analyzing a rack, the longer it takes, and so many systems can't be applied to hunting situations when the bulls is three steps from passing out of your life forever. If you have time, check the brow tines - do they appear to be out to the tip of his nose if you bent them out straight (they'll usually have an upcurl)? Still have more time? Another simple indicator is how the rack sweeps back. By that I mean, does the rack continue in basically a straight line as it passes the famous sword point? If so it's probably not a big bull. Really big bulls will usually have a pronounced angle of the main beam as it goes by the sword point.

I hope this helps, you're getting some good advice from a lot of the other guys on this post.