Step 1 - Counting:
While pedalling your bike, choose a foot and start counting every time it hits the bottom of the downstroke. You may count silently or aloud. I find that counting quite loudly works best, and helps clear busy multi-use pathways much better than an "on your left". Listen to the counting, and pretend that you are playing hide and seek and that you are 'it'.
Step 2 - Classify Yourself:
If you were one of the kids running off to hide, what would you think of the counting? Here are 5 main types of "counters" to help guide you in your cadence computation exercise:
Type 1: The Fair Kid
Type 2: The Competitive Seeker
If you are the Competitive Seeker type of cyclist, you are pedalling between 90 and 110 rpm, a good range for fitness and many other riding situations. If your goal was to really spin, however, you want to sound like our next Counting Type...
Type 3: The Big Fat Cheater
If you are a Big Fat Cheater (in a cycling cadence sense only, of course) you are spinning around 120 or even more... keep it up, and you will be the king or queen of spin, likely developing a smoother pedal stroke and sexy, supple leg muscles! Oooh baby.
Type 4: The Speed Freak
If your cadence count sounds like the Speed Freak, stop counting and get control of yourself and your bike, you maniac!
Type 5: The Confused Little One
If you sound like this, for the love of God, get off your bicycle and walk home if you can. You, my cycling friend, are drunk.
PS: While searching for hide and seek videos, I found this slightly disturbing tidbit, and y'know, why not share my mild discomfort?