There are four "standard" ways of measuring your BF%:
1) The dunk test. You get strapped into a harness and dunked into a pool. How much water you displace vs. how much you weigh determines your BF% (a lb of fat displaces more water than a lb of muscle -- fat floats because it's less dense). This is the most reliable, and the most expensive, method. Expect it to cost $75 - $125 or so.
2) Skin fold calipers. Measures how much flab fits between the tips of the calipers at multiple sites on the body -- the more flab, the higher the BF%. This is accurate
if done right. Easy to do wrong. Your doctor or gym may be able to do this (perhaps for a low fee - $10 or so). You can also buy a pair of calipers and attempt to do it yourself. A lot of detail on using calipers is here:
http://www.abcbodybuilding.com/magaz...stingguide.htm
3) Cloth measuring tape. Compares measurements taken with a cloth tape (available just about anywhere for a dollar or two) against your height and weight. An example site that does this is here:
http://www.healthcentral.com/cooltoo...s/bodyfat1.cfm
4) Inductance scale. Fat does not conduct electricity well; muscle does. So, you step on a scale with two electrical plates, and a current is sent through you. How well the current returns vs your height and weight determines your BF %. Tanita is the most well known scale maker of this type -- expect to spend $100+ for one. (The real problem with this method is your hydration level can wildly affect the reading, so it's best to measure things over time, rather than be able to measure your real BF% at a specific point in time.)