90-95 rpm cadence?

How is cadence counted? I don't think I spin that fast...more than once a second? That seems really fast. I've never measured it, and my 'puter doesn't count cadence. NOT a triathelete, mind you.

But I am curious about cadence. I can spin on the middle ring and do about 16 mph on flats and mash up hills on the big ring (couldn't do that 3 months ago), so I may be spinning faster than I think I am. How do I tell?

Karen