I don't really know what curves uses. There are a lot of different ways to check it, but few are very accurate. It might be a good idea to average a few different methods. I have used the scales and the hand held ones, they are basically the same thing. They send a current through your body. This current travels at a different speed through fat than it does through muscle. Thats how they come up with the result. These can be very inaccurate because there are too many variables. The caliper is a pretty good way to do it, but you need a fairly experienced person to do it to make sure they are pinching the right amounts in the right places. There are also formulas that can be fairly accurate. Be careful, some of these are dependant on BMI. Any one dependant on BMI could be very inaccurate. BMI is simply a ratio of height to weight and tells nothing of body composition. For example a male that is 6 feet tall and weights 210 lbs with 4% body fat would have the same BMI as someone that is 6 feet tall, weighs 210 lbs and has 20% body fat. I have an equation that is about as accurate as a formula can be. It is not dependant on BMI, rather, it is dependant upon body measurements. We are working on this page, if it doesnt work check back in a couple days. http://www.insiderfitness.com/calculators_forall.php
Your best bet is to do an average of different methods. If you cant do this, just use the same method everytime and note changes. The values might not be 100% correct, but as long as they are going down. you're in good shape.

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C Daniel
http://www.insiderfitness.com