If you are right handed:
Shoot a bare shaft into the target from about 20'.
If the shaft enters the target with the point to the right and the nock to the left, the shaft is too weak. If the point is to the left and the nock to the right, it is too stiff.
If the shaft is too weak you can correct by shooting a lighter point or by cutting the shaft shorter. If it is too stiff, you can shoot a heavier point, impossible to add length to the shaft. Or you can switch to a stiffer or weaker shaft if you can not correct as stated above. If you are left handed, the opposite is true on how the shaft enters the target. The easiest way to remember this is if the nock points towards your stronger arm it is too strong, if it points to your weaker arm it is too weak. A right hand person has a stronger right arm, left hand person has a stronger left arm. That is about as siple as I can give it.