  One of the most fundamental results of general relativity is that the event
horizon of black hole is purely ingoing. On the other hand,
semiclassical-gravity effects, such as particle creation and the quantization
of black-hole area, suggest that black holes can emit energy. Since a black
hole is characterized by the presence of the event horizon, the emitted energy
must be extracted from the black hole through its horizon. These considerations
provide a motivation to test the validity of the purely ingoing nature of
black-hole horizon. In this paper, we propose a novel test of the purely
ingoing nature of black-hole horizon through gravitational-wave detection. We
study the effects of hypothetical out-going gravitational waves to a perturbed
black hole by supplementing the boundary condition of gravitational waves at
the horizon with a phenomenological outgoing part. We show that this leads to
extra excitation of the usual quasinormal modes of a perturbed black hole and
continuous emission of gravitational waves. These additional signatures enable
us to test the boundary condition(s) of the black-hole event horizon through
gravitational-wave detection. Reanalysing the merger remnant of GW150914, we
constrain the intensity of the outgoing gravitational-horizon flux to be $ <
10^{40} \rm W$, which is roughly $ 10^{-9} $ of peak luminosity of GW150914.
