Identifying whether a shaft is ill fit to a golfer is a combination of both performance and FEEL issues.
When the shaft is too stiff for the golfer, most typically you see any or all of the following:
1. Loss of distance because the golfer cannot get the ball up as high to achieve the proper launch angle.
2. Possible tendency to leave the ball to the right
3. The feeling of impact between the ball and clubface feels harsh
4. The golfer may sense the shaft does not bend as much as he likes or would be used to, which in turn can trigger the golfer to swing harder to force the shaft to feel right, and from that comes more swing inconsistencies.
When the shaft is too flexible for the golfer, these are the things you would typically see:
1. Ball flight can be too high so the shot loses roll on the fairway and even shorter carry distance.
2. If the golfer already has a tendency to draw/hook the ball, the ball flight can hook/draw more from the same swing move.
3. The golfer may sense the shaft is bending too much which in turn can cause the golfer to think he is losing control, and/or cause the golfer to have to slow down/alter the swing to make the shaft's bending feel end up being less flexible feeling.
Shafts are a very fascinating part of the club because the only golfers who see the PERFORMANCE symptoms of a shaft being too stiff or too flexible are those golfers with a later to late wrist cock release and a slightly to more aggressive downswing tempo. But pretty much all golfers will note one of the two FEEL aspects of a shaft that is too stiff or too flexible. The most noted FEEL symptom experienced by golfers with a shaft too stiff is they sense the ball comes off the face with a more "dead feel" as if the shot wasn't hit that solid.