  Stellar-mass binary black hole mergers are poised to represent the majority
of gravitational-wave (GW) observations by Advanced LIGO and Virgo. Probing
their origin will be difficult due to the expected lack of electromagnetic
emission and limited localization accuracy. Associations with rare host galaxy
types -- such as active galactic nuclei (AGN) -- can nevertheless be identified
statistically through spatial correlation. We show that (i) fractional
contributions $f_{\rm agn}=50-100\%$ from AGN hosts to the total BBH merger
rate can be statistically established with 70-300 detected events (expected in
0.5-2 years of observation with Advanced LIGO-Virgo at design sensitivity and
current rate estimates); (ii) fractional contributions as low as $f_{\rm
agn}=25\%$ can be tested with 1000 events ($\sim$ 5\,years of observation);
(iii) the $\sim5\%$ best localized GWs drive these constraints. The presented
method and results are generally applicable to binary formation channels with
rare host populations.
