As Randy said, judge the horns with the ears. The average pronghorn has an ear length of 7" or so. Also, if you are close enough, and have good enough optics, the tips of the horns on a mature buck will have a yellow/whitish look to them. You want the prong of the horn to be above the tip of the ear by atleast 1/2", and you want as much horn above the prong as below it. These are a couple of things that I use and have found to work satisfactorily on getting +15" pronghorn bucks. I don't know what caliber your area is going to have, but good luck!