Mags such as Big Buck (produced in saskatchewan), pictures on websites/forums/blogs, videos are possible sources.

#1 tip forget about the body and neck sizewhile your judging the rack!!
Use visual aides such as:

Spread same as everywhere relaxed tip to tip runs 15-16" so compare inside spread.

Beams look for beams that sweep to the nose on a typical rack, will give you 22-25" beams(depending on the wrap). Look at the spacing between the G's(tines) if really tighton a 4 or 5x it means shorter beams. Ithen use the tine measurement aid below for such and as well on layed out beams or ones that you can't get a good feel for.

Tines: Visuals : Ave eye to nose measurement on a whitetail buck is 8-9" or the ears from base to tip ave. 6". Compare his tines (g's) or imagining them in this space will help you get an idea of tine length. In my mind unless a 4x is really high he will have a tough time taping 150" mark...so i study 4x's very closely as they can be deceiving. I have been on both sides of the coinwith 4x's.

Circum. (H's) mature deer in saskatchewan will pull 4" ave perH ( so its pretty much a given on a mature deer here has got 32-36" of mass on top). Though they can be much higher andlower. Comparing the base of the antler to the base of the deers ear can be used. the 4" base will be smaller then the ear base IME, if close to the ear base size they will be 5-6" bases indicating relaly good mass in 40" range for the deer. If bigger they are exceptional in mass and will see 45" or plus on the score sheet. This is by far the toughest attribute to judge in the field so the best way is looking at mounts and pictures to get a feel for a general rule of thumb to use.