  Supermassive black hole binaries may be detectable by an upcoming suite of
gravitational wave experiments. Their binary nature can also be revealed by
radio jets via a short-period precession driven by the orbital motion as well
as the geodetic precession at typically longer periods. We have investigated
Karl G. Jansky Very Large Array (VLA) and MERLIN radio maps of powerful jet
sources for morphological evidence of geodetic precession. For perhaps the best
studied source, Cygnus A, we find strong evidence for geodetic precession.
Projection effects can enhance precession features, for which we find
indications in strongly projected sources. For a complete sample of 33 3CR
radio sources we find strong evidence for jet precession in 24 cases (73 per
cent). The morphology of the radio maps suggests that the precession periods
are of the order of 10^6 - 10^7 yr. We consider different explanations for the
morphological features and conclude that geodetic precession is the best
explanation. The frequently observed gradual jet angle changes in samples of
powerful blazars can be explained by orbital motion. Both observations can be
explained simultaneously by postulating that a high fraction of powerful radio
sources have sub-parsec supermassive black hole binaries. We consider
complementary evidence and discuss if any jetted supermassive black hole with
some indication of precession could be detected as individual gravitational
wave source in the near future. This appears unlikely, with the possible
exception of M87.
