theres a couple ways.... if you go to a gym, ask them if they can do a skinfold test for you, thats basically using calipers. the scales your talking about would probably be impedance scales that get it from the electrical current running through your body which can be very innacurate because of all the variables.
i get mine done at a university that has a hydrostatic weigher, where they weigh you underwater, and through a bunch of calculations and other processess get your lean mass alone and get you a measurement accurate to 1-2%..... calipers are accurate 3-4%. and the impedance can be off by as much as 15 ive seen.