  The known exact solutions of Einstein's vacuum field equations modeling the
gravitational fields of pure gravitational radiation involve wave fronts which
are either planar or roughly spherical. We describe a scheme designed to check
explicitly whether or not the wave fronts collide. From the spacetime point of
view the scheme determines whether or not the null hypersurface histories of
the wave fronts intersect and, in particular, allows easy identification of the
cases in which the null hypersurfaces do not intersect.
