Best Answer - Chosen by Voters
If they are steel shafts and do not have a label with an S or S300 on it , then you can look at where the 1st step begins coming from the hosel. If that first step is within 4 or 5 inches ( on a 7 iron ) it is probably X, if the stp is between 6-8 in. , it is prob stiff , more than 8 inches prob reg. These are just estimations , every brand of shaft will have different measurements to first step. If they are graphite without any writing to indicate flex , try pushing grip end down to see how much and where it bends , if it bends really low, it is probably reg or senior , if it does not bend as easily and bends higher up the shaft it is probably stiff or X.
Source(s):
golf professional / fitter / repairman