There only truly accurate way to find out your swing speed is to measure it on a launch monitor as you hit balls out onto a range. Male tour professionals swing in the 115-120 mph range while average amateurs fall closer to the 80-85mph range. Fortunately, there are many ways to get access to a launch monitor, plus several other ways to estimate your swing speed closely enough to find the right clubs.
Finding your Swing Speed
Get Fitted for Clubs. When you buy new clubs, it is a good idea, regardless of where you end up purchasing the clubs, to visit a local PGA professional and get yourself fitted. He'll measure your body, give you a variety of clubs to hit and if he has one available, he'll put you on a launch monitor and swing speed is one of the statistics he'll collect. Some pro shops offer club fitting for free or as part of the price of a new set of clubs. Others offer it as a fee for service option. Talk to your pro and see what is available for you.
Take a lesson. The same approach applies as getting fitted for clubs. Talk to your local PGA pro about booking a lesson. If he has a launch monitor, he'll certainly measure your swing speed and help you out with any other problems you might be having with your game. Lessons can go for $35 per half hour all the way up to rather excessive charges depending on the professional and his or her reputation.
Try a Radar Monitor. There are several "radar monitor" devices on the market that claim to be able to capture your swing speed as you practice. Most of these devices are not particularly accurate, but they will give you a general idea of how fast you are swinging.
Use a Swing Speed Clip to estimate speed. Some manufacturers make a clip that you can fix to your club shaft near the head. This mechanical device registers swing speed on a mechanical scale, based on the force generated out toward the club head as you swing. Again, this will not give you a highly accurate measurement of how fast you swing a golf club, but it will give you a general feel for what range you might fall into.
Try a wide variety of clubs. Swing speed is really just one measurement used to determine what clubs are the best fit for your swing and your game. The most effective way to determine what clubs you should be playing is sometimes to ignore all the statistics and simply try a bunch of clubs at your local pro shop or driving range. Nothing tells you what works like actual live feedback.