Score is determined by the length along the outer curves and along four circumference measurements of each horn, so don't get fooled by width, especially when viewing from behind. Look for mass extending well out on the horns. A good rule of thumb: If the horn looks similar in width at the base and the midpoint, it's a good ram. You want a deep curl with the bottom of the horn dropping below the line of the lower jaw. The tips of the horns on mature rams are often ragged and broken, a condition called "brooming."
The best way to improve your field-judging skills is practice. Apply these tips while watching hunting videos, looking at magazines and surfing the Web. Of course, there is always the tried-and-true "holy @$#% method." If you don't say "Holy @$#%" when the buck strolls out, he's not big enough!
When judging a typical rack check for these three factors in this order: points, tine length and symmetry. Remember, 11 of the 19 scoring measurements on a typical 10-point buck are for length. You want at least 10 long points, with brow tines over 5 inches being a bonus. A buck's ear is about 6-7 inches long, so use it to check whether the G2s and G3s are at least as long.
Check the inside spread by comparing it with the 16- to 18-inch spread between an alert buck's ears. Anything past the ears is good. Missing tines and lop-sided racks really hurt the score, so view the rack from several angles if you have time. Antlers looking spindly in comparison to the ears have less mass. For non-typicals, look for everything above and then add in the extras.
A bull's torso is about 40 inches tall, so use it to check for main beam lengths of at least 30-40 inches. Likewise, a bull's chest is about 24 inches wide, so use it to measure inside spread, which should stretch at least 5 inches past the flanks on both sides. Having the opportunity to view a bull from several angles makes this much easier.
Mule deer tend to have shorter brow tines than whitetails, so longer ones really add to the score. Deep forks formed by G2s of at least 12 inches and nice-sized G3s distinguish mature animals from the "crab claws" of younger bucks. A high and wide rack appears boxy or square.