I'd like to add some to this subject. More often than not when you see a shooter class Buck it's usually in a situation where you only have seconds to determine it's size. everything I've read so far offers good tips and advice but in most cases there just isn't time to think "1,2 or 3 inches past ears, or antler tips almost to the nose or G1's G2's etc..
The Easiest way is to just become familiar with what a Rack looks like they're all pretty much the same, I mean, a spike is a spike (simple measure) "antler Height" a 3 or 4 pointer looks similar to other 3 or 4 pointers, then comes the Basket Racks 6's and 8's most basket racks look the same too. 10's 12's and larger, IN the woods are too obvious to hide their large size, so shooting one is just a "WOW It's BIG" method.
The decision starts before the hunt, you have to decide before you enter the woods if you want to shoot a Spike or not or whether a 4 pointer is fair game, a lot of hunters go hunting thinking they have loads of time to decide what to shoot, it isn't that way at all. so my advice is to
1. look at as many deer heads as possible, outdoor shows, mounts, fields etc.
(this will help you to know "Naturally" what type of deer is in front of you.)
2. decide before the hunt what type of deer will make you happy to shoot
even a spike can be a trophy to some hunters...
3. just enjoy the hunt don't worry about a deer's score eventually everyones
trophy comes along.
Good Luck and Happy Safe Hunting.