Computers are nice, but not necessary. I use one that doesn't have cadence capabilities. I just count the revolutions for 30 seconds, then double. Now I don't do that, since I know the feel of my cadence if I am pedaling too fast or slow. You can have a friend drive next to you and see how fast you are going. Remember this and what gear you are in, check Sheldon Brown's Gear Calculator and determine your cadence. More work, but can be done.

If you really want to do the math, you could use gear inches and speed to determine your cadence.