If you have a watch you can measure your cadence just by counting the number of times one leg pedals in a minute. You can maybe attach your watch to your handlebars to make it easier to see for that minute.

Whether or not you want a computer for your bike depends on what you want to do. I have all kinds of gadgets on my bike. I use the info I get from them to chart my progress and assess my training program. I can't imagine riding without them - even a fun ride.

Do you need all that? Not really

My husband generally has no gadgets on his bike. He doesn't care about his speed, cadence, HR... it doesn't matter. Except when I'm pulling him in a headwind, up a slight grade. Then he wants to know how fast we're going.

The article SK referenced in another post about cadence was really good.

V.