  Supermassive black hole binary systems (SMBHBs) emitting gravitational waves
may be traced by periodic light curves. We assembled a catalog of 149 such
periodic light curves, and using their masses, distances, and periods,
predicted the gravitational-wave strain and detectability of each binary
candidate using all-sky detection maps. We found that the International Pulsar
Timing Array (IPTA) provides almost uniform sky coverage -- a unique ability of
the IPTA -- and by 2025 will improve NANOGrav's current minimum detectable
strain by a factor of 6, and its volume by a factor of 216. Moreover, IPTA will
reach detection sensitivities for three candidates by 2025, and 13 by the end
of the decade, enabling us to constrain the underlying empirical relations used
to estimate SMBH masses. We find that we can in fact already constrain the mass
of a binary in Mrk 504 to $M<3.3\times 10^9~M_\odot$. We also identify 24
high-mass high-redshift galaxies which, according to our models, should not be
able to host SMBHBs. Importantly the GW detection of even one of these
candidates would be an essentially eternal multimessenger system, and
identifying common false positive signals from non-detections will be useful to
filter the data from future large-scale surveys such as LSST.
