Field judging bucks can be tricky, as a rule, one needs to determine height/width/mass/symmetry in relationship to the buck's ears and preferably from several angles, also factoring in deductions, broken tines, etc.. Through experience, it becomes much easier.
For practical experience, you could measure all of the bucks yourself and friends have bagged employing the B&C scoring system to gain some immediate feedback (linked here).
http://www.boone-crockett.org/ This would prove helpful, at a glance, to determine roughly what range a live buck might fall into (120's, 150's, etc..).
I will say, unless I'm hunting a specific buck or two, or have ample time to be selective on awesome ground...I merely limit myself to what I refer to as 'keeper' bucks (bucks any reasonable deer hunter would be thrilled with).
If you were to become consumed by field judging/limiting yourself to only those bucks appraoching 'book proportions', it can sometimes suck the life right out of you and leave you with several, if not many, 'dry seasons'. Not to mention missing opportunities by field judging when you should be concentrating on the wind, the buck's demeanor, getting the gun up without being seen, what spot you intend to shoot him in relation to the surroundings, the shot itself, etc.. Once I establish a buck a 'keeper', my mind is totally off the rack from that point on...till he's on the ground.