  Information about the collapsed matter in a black hole will be lost if
Hawking radiations are truly thermal. Recent studies discover that information
can be transmitted from a black hole by Hawking radiations, due to their
spectrum deviating from exact thermality when back reaction is considered. In
this paper, we focus on the spectroscopic features of Hawking radiation from a
Schwarzschild black hole, contrasting the differences between the nonthermal
and thermal spectra. Of great interest, we find that the energy covariances of
Hawking radiations for the thermal spectrum are exactly zero, while the energy
covariances are non-trivial for the nonthermal spectrum. Consequently, the
nonthermal spectrum can be distinguished from the thermal one by counting the
energy covariances of successive emissions, which provides an avenue towards
experimentally testing the long-standing "information loss paradox".
