Take the knuckle of your dominate hand (i.e. if you are right handed use it, if you are left handed use it) and place it behind your cheek bone on the same side. Extend your other arm all the way out in front of you like you are drawing a bow and then have some one measure that distance. You can add about an inch to that and be fairly close and this will give you a really good idea. It will not be exact, though. Your best bet however is to do just as you plan to do and go to your local pro shop.

Good hunting.