You can count cadence without a special computer. All you need is a time keeping device that will let you know when 30 seconds has passed, and then start counting. Everytime your right foot reaches the bottom of the pedal stroke count another stroke. Then multiply your total by 2 for revolutions per minute (RPM).

Generally speaking, if you're racing you should aim for a fairly high cadence (above 90). If your goal is to do longer distance rides, aim for a cadence between about 80 and 90 ... recreational rides would probably fit into that range as well.

A higher cadence (i.e. above 80) is easier on the knees, especially over longer distances.