three major things I try to notice for a deer are

front shoulder area / if you can tell that the majority of the weight is upfront with large chest mass it is most likely a mature deer

body size / you just know when it is a big deer

antler mass / there are some 2 1/2 - 3 1/2 yr olds with nice racks but they haven't really developed any thickness to them yet

I try to look at these things while fighting off the buck fever. Years ago I would just panic and think the deer will turn and walk away before I can get off a shot. Last 3 years I passed a lot of smaller bucks for mature ones and it has paid off 2 of 3. There aren't many large deer in MA on a regular basis so it is hard to let them go. A lot of year round scouting and time in the tree stands, but it works