  While recent progress in the black hole information problem has shown that
the entropy of Hawking radiation follows a unitary Page curve, the quantum
state of Hawking radiation prior the Page time is still treated as purely
thermal, containing no information about the microstructure of the black hole.
We demonstrate that there is significant quantum information regarding the
quantum state of the black hole in the Hawking radiation prior to the Page
time. By computing of the quantum fidelity in a 2D boundary conformal field
theory (BCFT) model of black hole evaporation, we demonstrate that an observer
outside of an evaporating black hole may distinguish different black holes via
measurements of the Hawking radiation at \textit{any} time during the
evaporation process, albeit with an exponentially large number of measurements.
Furthermore, our results are universal, applicable to general BCFTs including
those with large central charge and rational BCFTs. The techniques we develop
for computing the fidelity are more generally applicable to excited states in
CFT. As such, we are able to characterize more general aspects of
thermalization in 2D conformal field theory.
