Here are a few things that will help you quickly estimate a gross score on a buck in the field.
1. The ear spread, for an alert buck is 16 to 18 inches, so if the main beams are at least as wide as the ears you know you've got at least 16-18 inch spread.
2. A deers ear is between 6-8 inches, so you can use that to estimate tine length.
3. An antler that curves in at the tip rather than staying straight indicates a long main beam.
4. The circumference of a deer's eye is approx. 4 inches. You can quickly estimate the mass of the antlers by comparing them to the circumference of the deers eye.
Check out this website
http://www.shanepotter.com/fieldjudging.html it goes over what I've already stated plus gives you a couple of other pointers.