  The amoeba of an affine algebraic variety V in (C^*)^r is the image of V
under the map (z_1, ..., z_r) -> (log|z_1|, ..., log|z_r|). We give a
characterisation of the amoeba based on the triangle inequality, which we call
testing for lopsidedness. We show that if a point is outside the amoeba of V,
there is an element of the defining ideal which witnesses this fact by being
lopsided. This condition is necessary and sufficient for amoebas of arbitrary
codimension, as well as for compactifications of amoebas inside any toric
variety. Our approach naturally leads to methods for approximating hypersurface
amoebas and their spines by systems of linear inequalities. Finally, we remark
that our main result can be seen a precise analogue of a Nullstellensatz
statement for tropical varieties.
