A proper gauge for determining what shaft a golfer should use is swing speed. Swing speeds below 85 mph would be best for regular shafts while anything above would use stiff shafts. Using the proper shaft will help the golfer to have proper ball flight and increased distance.
Going to a professional is probably the best bet when trying to get fitted for the proper shaft flex with new clubs. Each type of club is a little different, which may affect the way the club swings and the proper shaft for that swing.
The club must have a good feel to the owner. If you are seeing negative results with a club, consult a club maker or pro golf shop to see if the shaft is what's best for you.
Show caution when buying used clubs; many sellers do not reveal what type of shafts the clubs have, which may be important to the purchase. Always check before buying.
Practice with the clubs as much as possible. If you are switching to new clubs with unfamiliar shafts, it may take some repetition before it feels correct. If properly fitted, the clubs should maximize your golf game.