Watch as many elk hunting shows as you can on TV.
They will generally give you the approximate score of the animal they shoot and it can help alot. Also sometimes they will comment on some of the ones they pass on and give an estimated score on those. The tactics are often not very useful as they generally hunt low pressure private land, but you get to see plenty of elk antlers to judge. There are a couple shows with public land elk hunting that can actually be useful though.
400" is a total monster that will have you falling all over yourself, there would be no doubt in your mind about if it was a shooter or not. For most folks 350" is a serious trophy on public land and you won't have much doubt on one of those either. For me personally 320"ish is what it is going to take to go on my wall as a full mount and that's a good solid 6x6 with good mass and decent spread. Tine length is important as previously mentioned.
300" will get you a 6x6 withdecent mass and a few short tines or a really good 5x5 with great mass and good tine length canget to 300" also. This would be a decent elk for most and I personally have never shot a bull elk so it would be hard for me to pass up any 6x6 or a good 5x5.
250" - 300" is going to be a weaker 6x6 and medium to weak 5x5's. These are going to have broken or very short tines and limited mass and spread. These still go on some folks walls but usually as a european type mount.
That's my basic laymans scoring effort.
I found this site that has quite a few pictures of elk with the score under the picture - http://www.kingsoutdoorworld.com/trophy-room/elk-bb.shtml