  The key difference between neutron stars and black holes is the
presence/absence of a solid surface. Recent attempts to detect this difference
have concentrated on the quiescent luminosity, but here these sources are faint
and difficult to observe. Instead we look at these sources when they are
bright, and show that there is a clear difference between black holes and
neutron stars in the evolution of their X-ray spectra which is due to the
presence of a surface in the neutron stars. We also show that there is a type
of X-ray spectrum which is only seen from black holes, making it a good
diagnostic for the nature of new transient sources.
