
On the opposite spectrum, working out at a low pulse rate will not benefit you as much as working in your target heart rate zone.
There are two simple ways to check your pulse. One is manually. This site does a very nice job of describing ways to check your pulse using your fingers.
Another way to monitor your heart rate is with an electronic heart rate monitor. These usually consist of a band that straps around your chest (females, under your breasts) and a coordinating wrist watch that reads from the monitor on your chest.
Heart rate monitors can get pricey, so unless you have heart problems, or exercise frequently, usually checking your pulse rate manually will do. My favorite most trusted brand of heart rate monitors are from the Polar line, but here is another link of monitors for under $100.