  Distinguishing whether a system supports alternate low-energy (locally
stable) states -- stable (true vacuum) versus metastable (false vacuum) -- by
direct observation can be difficult when the lifetime of the state is very long
but otherwise unknown. Here we demonstrate, in a tractable model system, that
there are physical phenomena on much shorter time scales that can diagnose the
difference. Specifically, we study the spectral density following a quench in
the tilted quantum Ising model, and show that the evolution of the spectral
density is a powerful diagnostic. Small transition bubbles are more common than
large ones, and we see characteristic differences in the size dependence of
bubble lifetimes even well below the critical size for false vacuum decay. We
expect this sort of behavior to be generic in systems of this kind. We show
such signatures persist in a continuum field theory. This also opens the
possibility of similar signatures of the potential metastable false vacuum of
our universe well before the beginning of a decay process to the true vacuum.
