  We consider a sonic black-hole scenario where an atom condensate flows
through a subsonic-supersonic interface. We discuss several criteria that
reveal the existence of nonclassical correlations resulting from the quantum
character of the spontaneous Hawking radiation. We unify previous general work
as applied to Hawking radiation analogs. We investigate the measurability of
the various indicators and conclude that, within a class of detection schemes,
only the violation of quadratic Cauchy-Schwarz inequalities can be discerned.
We show numerical results that further support the viability of measuring deep
quantum correlations in concrete scenarios.
