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 case of neutron stars. We also show that there is a type of X-ray spectrum that is only seen from black holes, making it a good diagnostic for the nature of new transient sources.
Done, C., & Gierliński, M. (2003). Observing the effects of the event horizon in black holes. Monthly Notices of the Royal Astronomical Society, 342(4), 1041-1055. https://doi.org/10.1046/j.1365-8711.2003.06614.x