Neutron stars are supposed to have an upper limit in mass, beyond which they collapse into perhaps a further type of degenerate matter or a singularity. There doesn't seem to be precisely defined upper limit, but the limit is estimated to be 1.5-3 solar masses. However, a hypothetical neutron star with a mass greater than about 3 solar masses would fit inside its schwarzschild radius, making it a black hole from our perspective. So why do physicists think that neutron stars collapse any further? It seems like there would be no way of observing whether a black hole is composed of a massive neutron star or a singularity?