Difficult to explain and make any sense, but here goes! One method is to hold your arm out so that the elbow makes a 90* angle between your upper and fore-arm. Point your trigger finger and measure from the crook of your elbow to the point on the trigger finger where you like to engage the trigger. It is often easier if you use a broom handle and pretend its a rifle, grab the imaginary stock wrist and measure to the bent trigger finger...
You can also do this holding a rifle and not grasping the pistol grip. If I wrap my elbow around the but of the rifle, my hand and trigger finger naturally fall further forward than the pistol grip and trigger of most factory rifles and I can quickly estimate how much extra butt length would be needed for me.
A better way would be to borrow a target rifle with adjustable butt plate and measure off that.
A friend of mine swears that he likes short LOP but I'm sure he is just in denial (and I outshoot him!)