If you want to know your bodyfat percentage, and you want it to be accurate, a reliable way is to get hydrostatically weighed. I did this by tracking down a grad student at one of my local universities and made an appointment with him. There are a couple of other newer ways that are extremely accurate but I don't know much about them.
Calipers, though popular, are not very reliable unless you can get someone who has extensive experience and practive using them. Most people at health clubs are not even qualified enough to do this accurately unless they've done thousands of tests. But calipers can be useful. While you won't likely be able to accurately measure your actual bodyfat with them, as long as you're measuring at the exact same spot on your body every time, you can at least tell whether you're gaining or losing bodyfat.
Bioelectrical impedence testing can be fairly accurate, but the pre-test requirements, if not followed, can skew the result by several percentage points.
Marine, what you may be talking about is the Body-Mass Index, which I wouldn't even bother doing because it's not even a measurement of bodyfat. At just above 6 feet and around 235 I'm considered obese according to that index and I should seek medical attention immediately. That's complete crap because I know that my bodyfat is around 15%.
BC.