I haven't yet done a spinning class (no need here, since this 'winter' has been a joke so far), but I believe the indoor bikes have timers? Just count your pedal strokes for 15 sec and multiply by 4. That'll give you your cadence (+/- 4 strokes). Every time your left foot bottoms out is a pedal stroke. Or whatever part of the cycle is easiest. This is what I do when I'm on the road. It might be a little harder with the music going...