Birth certificate. Or stop the deer, take one of his teeth out, and do an analysis of it. The birth certificate's easier!
Seriously, the best you can do is guess, based on the deer's body size and antler MASS. Not points or spread, but mass. And if you can get a good look at the bases of the antlers, that's even better. But to even make a reasonable guess this way you have to know the deer in the area that you are hunting, and what's "average" for that area for body size and antler mass.
At deer check stations they will look at the deer's molars to get an age guess, but as I said, the only sure way is to do an analysis of a tooth.
IMO it's kind of bogus for them to put an age minimum of what bucks they will allow you to kill, when there's no sure way of determining that until the deer's dead (unless you can get one to agree to stop and let you look in his mouth!). In fact, what if you kill one that to YOU sure as shootin' looks like it's FIVE years old, and they say that no, according to them it's only 2 1/2. Who's opinion is right? A better way is to have an antler point minimum and/or a spread minimum (although as hard as I've tried, I could NEVER get a buck to hold still long enough for me to get a tape on him before I shot him!)
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