In order to use a golf ball in tournaments and official competitions, it must conform to specific regulations set in place by the United States Golf Association. For example, the PGA Tour utilizes USGA guidelines in Appendix III to determine what is and is not legal for play in each competition. Before being put on the market, every golf ball undergoes extensive testing based on color, material, striking distance and manufacturer code.
Size: Officially, a ball must weigh no more than 1.620 oz. and have a diameter no less than 1.680 inches. Furthermore, it must be spherically symmetrical and cannot exceed the USGA's sanctioned Initial Velocity limit.
Materials: A competition golf ball cannot be constructed using outside materials that could increase a golfer's performance; nor can it possess any cuts, cracks, dents or deformities that may potentially affect the ball's aerodynamics. Similarly, the balls are tested for modified elements that could induce greater compression and spin rates.
Colors: Even though a majority of golf balls permitted by the USGA have a cover color of white, the committee has allowed the usage of other bright colored golf balls during official competition.
Distance: To keep the game fair, the USGA restricts flight and subsequent ground-roll distance. This is often tested alongside new developments golf technologies.
If you wish to determine whether a golf ball is legal or not, you can always refer to www.usga.org or www.randa.org. These websites contain the "Conforming Golf Balls" file, which is a 35-page document comprised of over 1,000 golf ball models that have been accepted by the USGA for fair play.
While using an illegal golf ball may improve your driving distance by 35 yards, the consequences of are far worse than the benefits. In the most extreme cases, playing with an illegal ball may lead to disqualification and poor reputation. So play fair and avoid them at all costs.