Since there is no standard for regular or stiff, it doesn't hurt to experiment with different weights and advertised flex in a fitting.  Without knowledge of your swing, more than half the golfers with a stiff/XS flex shaft would do better with a softer flex.  Willing to bet most of those with the wrong flex don't get fit, either.  At the same swing speed, the same shaft in a heavier weight will have a lower torque and promote less spin.  The right shaft is the one that gets the best results, regardless of flex or weight.

Don gave a great answer but to answer the question in the post title...  Youl'll know when the driver feels like it was shafted in rebar flex