Tooth wear once in your hand. In the field posture(sway back and or sagged belly), size of legs to body, forehead looks tuffed or swollen, roman nose - dropped and demeanor. A mature buck walks the walk and talks the talk...he's king in his mind and wears it with pride. Body size in terms of girth and thickness can be another indicator, however big body genetics are normal (here) so it wouldn't by my first way to try and put a maturity mark on him. Great way is history, knowing the bucks in your area and watching them develop but you'll always get those where did he come from....

Antler size not so much, while maturity will allow that deer to reach its full potential, genes, food and stress play a big part in year to year antler growth. IE: world record archery mule deer buck was taken in 2011 here, he put on over 70"'s of antler in one season...sheds from previous year put him at 200" mark. Crazy!!!