  Distinguishing conformally coupled frames from the tree-level perturbative
observables (scalar spectral index $n_{\rm s}$ and tensor-to-scalar ratio $r$)
is challenging in cosmology as they are nearly identical. However, since the
background evolution in these two frames differs significantly, we can look for
potential signatures in the reheating constraints to discriminate these frames.
In this work, we study the reheating phase in these frames and find that the
difference in the inflationary energy scales in these frames contributes to a
significant difference in the reheating e-folding number and hence, different
reheating temperature. This difference will eventually lead to a contrasting
thermal history in the two frames, which may have a potential observational
signature in future observations. This study will open up an avenue for
distinguishing various conformally connected otherwise indistinguishable frames
and may finally lead us to the correct theory of gravity for our Universe.
