  In this paper, we derive a simple equality that relates the spectral function
$I(k,\omega)$ and the fidelity susceptibility $\chi_F$, i.e. $%
\chi_F=\lim_{\eta\rightarrow 0}\frac{\pi}{\eta} I(0, i\eta)$ with $\eta$ being
the half-width of the resonance peak in the spectral function. Since the
spectral function can be measured in experiments by the neutron scattering or
the angle-resolved photoemission spectroscopy(ARPES) technique, our equality
makes the fidelity susceptibility directly measurable in experiments.
Physically, our equality reveals also that the resonance peak in the spectral
function actually denotes a quantum criticality-like point at which the solid
state seemly undergoes a significant change.
