  An experiment is proposed to test a previously developed theory of the
hydrodynamics of a nonequilibrium heat current-induced superfluid-normal
interface. It is shown that the interfacial ``trapped'' second sound mode
predicted by the theory leads to a sharp resonant dip in the reflected signal
from an external second sound pulse propagated towards the interface when its
horizontal phase speed matches that of the interface mode. The influence of the
interface on thermal fluctuations in the bulk superfluid is shown to lead to
slow power dependence of the order parameter, and other quantities, on distance
from it.
