  We show that measuring commuting observables can be sufficient to assess that
a bipartite state is entangled according to either nonseparability or the
stronger criterion of 'steerability'. Indeed, the measurement of a single
observable might reveal the strength of the interferences between the two
subsystems, as if an interferometer were used. For definiteness we focus on the
two-point correlation function of density fluctuations obtained by in situ
measurements in homogeneous one-dimensional cold atomic Bose gases. We then
compare this situation to that found in transonic stationary flows mimicking a
black hole geometry where correlated phonon pairs are emitted on either side of
the sonic horizon by the analogue Hawking effect. We briefly apply our
considerations to two recent experiments.
