  It is often the case that the environment of a quantum system may be
described as a bath of oscillators with Ohmic density of states. In turn, the
precise characterization of these classes of environments is a crucial tool to
engineer decoherence or to tailor quantum information protocols. Recently, the
use of quantum probes in characterizing Ohmic environments at zero-temperature
has been discussed, showing that a single qubit provides precise estimation of
the cutoff frequency. On the other hand, thermal noise often spoil quantum
probing schemes, and for this reason we here extend the analysis to complex
system at thermal equilibrium. In particular, we discuss the interplay between
thermal fluctuations and time evolution in determining the precision
{attainable by} quantum probes. Our results show that the presence of thermal
fluctuations degrades the precision for low values of the cutoff frequency,
i.e. values of the order $\omega_c \lesssim T$ (in natural units). For larger
values of $\omega_c$ decoherence is mostly due to the structure of environment,
rather than thermal fluctuations, such that quantum probing by a single qubit
is still an effective estimation procedure.
