  The scarce knowledge of the initial stages of quark-gluon plasma before the
thermalization is mostly inferred through the low-$p_\perp$ sector. We propose
a complementary approach in this report - the use of high-$p_\perp$ probes'
energy loss. We study the effects of four commonly assumed initial stages,
whose temperature profiles differ only before the thermalization, on
high-$p_\perp$ $R_{AA}$ and $v_2$ predictions. The predictions are based on our
Dynamical Radiative and Elastic ENergy-loss Approach (DREENA) framework. We
report insensitivity of $v_2$ to the initial stages, making it unable to
distinguish between different cases. $R_{AA}$ displays sensitivity to the
presumed initial stages, but current experimental precision does not allow
resolution between these cases. We further revise the commonly accepted
procedure of fitting the energy loss parameters, for each individual initial
stage, to the measured $R_{AA}$. We show that the sensitivity of $v_2$ to
various initial stages obtained through such procedure is mostly a consequence
of fitting procedure, which may obscure the physical interpretations. Overall,
the simultaneous study of high-$p_\perp$ observables, with unchanged energy
loss parametrization and restrained temperature profiles, is crucial for future
constraints on initial stages.
