The only sure fire way I know of is to use the teeth. You can make an educated guess based on body size and mass for your particular area, antlers, or other factors like the size of the chest or a sway belly. For me I use that and other factors. If I'm meat hunting I look for any deer with an injury. If it's the first three weeks of the season I look for barren does, or does that I think because of body mass or other factors might be older, but then only take one doe per hunting location (or herd if I think there's more than one). If it's late Oct I pass on all does till after the rut. I specifically pass on fawns out of spots, yearlings, button bucks, and anything 6 points or less, or 8 points if it's a basket 8. Having said that if I think it might be a cull buck I might take them....for example I've taken a small basket 8 where one side of the rack was stunted and funky, or a spike that had spikes over a foot and never branched......I've had buds take 6 pointers that had such a big body in their judgement they should have been 8's (the assumption being they needed to be taken out of the gene pool since they don't have better antler development....they usually see these several time a season), or 4 or 6 pointers that were maimed...like missing a leg. But the objective is to take does for herd management and a good buck that is an 8 pointer or more and is 3.5 years old or older.

Now having said all that..............if it's been dry for a while and I'm worried about my additude I'll take a doe for meat (my wife's girl fiends have standing orders) just to keep my attitude intact....but I'll always let 4 pointers and spikes pass all day long.....is that a complicated decision matrix for ya? It's more complicated than how I pick women.......;-)