  The relativistic spin-precession equations for black-hole binaries have four
different equilibrium solutions that correspond to systems where the two
individual black hole spins are either aligned or anti-aligned with the orbital
angular momentum. Surprisingly, it was demonstrated that only three of these
equilibrium solutions are stable. Binary systems in the up-down configuration,
where the spin of the heavier (lighter) black hole is co- (counter-) aligned
with the orbital angular momentum, might be unstable to small perturbations of
the spin directions. After the onset of the up-down instability, that occurs
after a specific critical orbital separation $r_\mathrm{UD+}$, the binary
becomes unstable to spin precession leading to large misalignment of the spins.
In this work, we present a Bayesian procedure based on the Savage-Dickey
density ratio to test the up-down origin of gravitational-wave events. We apply
this procedure to look for promising candidates among the events detected so
far during the first three observing runs performed by LIGO/Virgo.
