Carbonfiberboy 

Very good question. Well, the first thing to do is to watch it while you're riding and see what happens when you vary cadence, sorta like when one gets their first HRM. Try to figure out what the numbers mean, specifically what they mean for your speed and endurance. What's your best cadence for long efforts on the flat, what's your best cadence for short climbs, long climbs, medium climbs. If you have a PM, it makes this process a little quicker, but not much.

An easy thing to do is to ride a long climb, say 2000', and vary your cadence by fairly large numbers, say 10 rpm, holding same power or speed in each cadence for say 10', and see what your legs and lungs do. See what range seems best, then try to refine that a bit more. Do that in the saddle and standing.

A good way to decide on best cadence in any situation is that your legs and lungs should hit their limits at about the same time. If you run out of lungs but legs are still good, lower your cadence. If your legs give out but you still have reserves in your breathing, raise your cadence.

Then when you see what your best cadence numbers are, you can go on Bikeforums and see what other people's best cadences are and then ponder that over the years, maybe try to train at other cadences for a while, see if that makes a difference to your endurance, and therefore overall speed in various situations. Always keep investigating.

Once you figure out what your best cadences are in various situations, keep an eye on your cadence as you ride, particularly on long or intense rides, making sure you are using your best cadences even as you tire.