OK, so how does my buddy know what a buck would score after catching only a glimpse of it? Like most skills in life, experience and practice are the best teachers. To begin, commit these facts to memory:
The spread from ear tip to ear tip for an alert buck is about 16-17 inches. A 125-class buck usually has an antler spread equal to or a bit wider than his ear-to-ear spread.
A deer's ear measures 6-8 inches long. A 125-class buck usually has at least two points (not including brow tines) on each main beam that are as long or longer than his ears.
Finally, attend a whitetail hunting show in your area. Walk the aisles and keeping the above facts in mind, estimate the score of various trophy mounts before you look at their actual score. Add or subtract points (inches) from a base score of 125, then check your guess for accuracy.
In a short period of time, you'll begin to identify the "look" of various size racks, and the next time you see a buck in the wild, his score won't be a mystery.
So what about the buck in this photo? I passed him while hunting in Alabama, and I estimated his score at 120. Ten minutes later a slightly larger buck appeared, and I anchored him with one shot. He scored exactly 125.