Nails on 7/19/2007 5:42:33 PM:
You'll need a bike computer that has a cadence option. Models avalable include:
Cateye: Strada Cadence, CC-CD300DW, and Astrale 8.
Topeak: Comp140.
Vetta: VL110 T2X, VL110A T2X, VL110HR T2X, V100 WL2X, V100A WL2X, V100HR WL2X, and RT77.
Or you could really geek it by getting one of the Polar heart rate monitors designed for cycling (S725x/S625x, CS400, CS300, SC200cad, CS200, CS100b, and CS100) with the cadence option.
After a while the cadence becomes second nature. Enjoy!