  We propose a spectrometric method to reconstruct the motional states of
mechanical modes in optomechanics. This is achieved by detecting the
single-photon emission and scattering spectra of the optomechanical cavity.
Owing to an optomechanical coupling, the \emph{a priori} phonon-state
distributions contribute to the spectral magnitude, and hence we can infer
information on the phonon states from the measured spectral data. When the
single-photon optomechanical-coupling strength is moderately larger than the
mechanical frequency, then our method works well for a wide range of
cavity-field decay rates, irrespective of whether or not the system is in the
resolved-sideband regime.
