  Recent discoveries of black hole (BH) candidates in Galactic and
extragalactic globular clusters (GCs) have ignited interest in understanding
how BHs dynamically evolve in a GC and the number of BHs ($N_{\rm{BH}}$) that
may still be retained by today's GCs. Numerical models show that even if
stellar-mass BHs are retained in today's GCs, they are typically in
configurations that are not directly detectable. We show that a suitably
defined measure of mass segregation ($\Delta$) between, e.g., giants and
low-mass main-sequence stars, can be an effective probe to indirectly estimate
$N_{\rm{BH}}$ in a GC aided by calibrations from numerical models. Using
numerical models including all relevant physics we first show that
$N_{\rm{BH}}$ is strongly anticorrelated with $\Delta$ between giant stars and
low-mass main-sequence stars. We apply the distributions of $\Delta$ vs
$N_{\rm{BH}}$ obtained from models to three Milky Way GCs to predict the
$N_{\rm{BH}}$ retained by them at present. We calculate $\Delta$ using the
publicly available ACS survey data for 47 Tuc, M 10, and M 22, all with
identified stellar-mass BH candidates. Using these measured $\Delta$ and
distributions of $\Delta$ vs $N_{\rm{BH}}$ from models as calibration we
predict distributions for $N_{\rm{BH}}$ expected to be retained in these GCs.
For 47 Tuc, M 10, and M 22 our predicted distributions peak at
$N_{\rm{BH}}\approx20$, $24$, and $50$, whereas, within the $2\sigma$
confidence level, $N_{\rm{BH}}$ can be up to $\sim150$, $50$, and $200$,
respectively.
