  We propose a simple test of underground neutrino data to discriminate amongst
neutrino oscillation models. It uses the asymmetry between downward-going
events and upward-going events, for electron and muon events separately.
Because of the symmetry of typical underground detectors, an asymmetry can be
compared with calculations with little need for the intermediary of a
simulation program. Furthermore, we show that the various oscillation scenarios
give rise to dramatically differing trajectories of asymmetry versus energy for
muons and electrons. This permits a clean distinction to be drawn between
models.
