By Liz Neporent

Consider two women who stand 5-foot-5 and weigh 140 pounds. One is a lean marathon runner while the other is an out of shape couch spud. The scale won't be able to tell the difference between these two but a measure called body fat percentage can.

Body fat percentage estimates how much of your body is composed of fat and how much is composed of muscle and other lean body tissues. Experts generally agree that a healthy body fat range for men is 12-20 percent and for women, 16-26 percent; women get a few extra points grace for the essential "reproductive" fat they carry.

Estimating your body fat isn't all that complicated but different methods vary in terms of cost, convenience and accuracy. Here are the five most common ways to measure your fat.

Pinch an Inch
The most widely used body fat test involves skinfold calipers, a gizmo that looks like a cross between salad tongs and a stun gun. The tester, usually a personal trainer, pinches your skin in three to seven places to pull the fat away from your muscles and bones and estimate your fat-to-muscle ratio. If your pincher is unskilled, you'll get a number that's off by up to 6 percent so ask if your tester has proper training. Some gyms provide caliper testing for free when you buy a membership or a personal training package but if the offer to pinch comes before you sign up, beware: Some unscrupulous salespeople will intentionally skew the results upwards to make you think you're fatter than you are.

Get Zapped
Another widespread body fat measure called bioeletrical impedance is usually offered for free or for a low fee at gyms and health fairs. With this test you'll either step onto a platform that looks like a scale or grab onto a steering wheel-like gadget which sends a mild electrical current through your body. The slower the signal, the more fat you have because fat impedes, or blocks, the signal more than muscle. While simple, the technique is error prone especially if you're extremely overweight or lean. Being dehydrated can also slow the signal down so you appear to have more fat than you really do.

Go Under
Underwater weighing is cumbersome, but highly accurate. Based on the basic premise that fat floats and muscle sinks, you sit on a special weight scale in a tank of warm water, blow all the air out of your lungs and then bend forward until you're completely submerged. After about five seconds, the machine calculates your body fat. The margin of error is about 2 percent for all but very lean, very overweight or old adults. Those willing to brave the water can get dunked at sophisticated sports medicine clinics or labs for $50 to $150.

Air Out
Research suggests that a relatively new test using a large egg-shaped fiberglass chamber called the BOD POD is just as accurate as underwater weighing. For this test, you're encased in a chamber for 20 seconds while computerized pressure sensors determine how much air your body displaces. In other words, how much space you take up. That gets translated into a body fat percentage. A handful of university research labs and health clubs around the country offer this service around $50.

Go Techno
Dual-Energy X-ray Absorptiometry, or DEXA, measures not only how much fat you have but also where on your body the fat is located -- a more relevant health indicator. Originally developed to scan bone density, you lie on a bed while a low dose of two different x-ray energies scan your body from head to toe. DEXA is still leading edge but available at some hospitals and doctors' offices for around $300, and usually requires a physician's referral.