  Compact Galactic binary systems with orbital periods of a few hours are
expected to be detected in gravitational waves (GW) by LISA or a similar
mission. At present, these so-called verification binaries provide predictions
for GW frequency and amplitude. A full polarisation prediction would provide a
new method to calibrate LISA and other GW observatories, but requires resolving
the orientation of the binary on the sky, which is not currently possible. We
suggest a method to determine the elusive binary orientation and hence predict
the GW polarisation, using km-scale optical intensity interferometry. The most
promising candidate is CD-30$^{\circ}$ 11223, consisting of a hot helium
subdwarf with $m_B = 12$ and a much fainter white dwarf companion, in a nearly
edge-on orbit with period 70.5 min. We estimate that the brighter star is
tidally stretched by 6%. Resolving the tidal stretching would provide the
binary orientation. The resolution needed is far beyond any current instrument,
but not beyond current technology. We consider scenarios where an array of
telescopes with km-scale baselines and/or the Very Large Telescope (VLT) and
Extremely Large Telescope (ELT) are equipped with recently-developed kilo-pixel
sub-ns single-photon counters and used for intensity interferometry. We
estimate that a team-up of the VLT and ELT could measure the orientation to
$\pm 1^{\circ}$ at 2$\sigma$ confidence in 24 hours of observation.
