  Although squeezed states are nonclassical states, so far, their
nonclassicality could not be demonstrated by negative quasiprobabilities. In
this work we derive pattern functions for the direct experimental determination
of so-called nonclassicality quasiprobabilities. The negativities of these
quantities turn out to be necessary and sufficient for the nonclassicality of
an arbitrary quantum state and are therefore suitable for a direct and general
test of nonclassicality. We apply the method to a squeezed vacuum state of
light that was generated by parametric down-conversion in a second-order
nonlinear crystal.
