In addition to just using the “personal feel” test (i.e. do you feel like you’re able to create the proper amount of “whip” when you take a full golf swing)… you will predominately be able to tell that you are in need of a more flexible/less stiff shaft if all (or most) of your misses tend to be low and to the right of your intended target line (for a right-hander).  Conversely, if it is more common for you to miss high or to the left of the target, a shaft that is too light and too flexible is more likely to blame.