Now that's a direct question! I use a ruler! Sorry, I know, that was flipant. The pull length is the length at which the shooter's trigger hand elbow will measure a 90 degree bend with the weapon properly6 mounted and the trigger finger engaged. Of course cast off or cast on as well as stock design can alter that some but that's a general rule. Short pulls on straight gripped guns can lead to getting the back of the trigger hand's thumb in the nose or a scope to the eyebrow. Many competition trap shooters like extremely long pulls to help build in lead for the birds. Measuring pull is easy, use a rigid measuring device and measure from the center of the butt plate to the deepest part of the trigger arch. Measuring the pull length is quite easy, determining what the right pull length is for someone is much more difficult. Even the British fitters use try guns, which is in the end a very good idea. Just a gun with a nearly infinitely adjustable stock.. then they measure..