Here's one way I can think of: lay down and put your head only on a bathroom scale. That should get just about most of the weight. You'll need either a mirror or another person to actually be able to see the weight, though.
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I think another way is to do with getting into a bath full of water (of known volume, of course). Then you see how much water is displaced by the whole of your body, and then put just your head in to check displacement. And then it's a calculation.