It is not possible to know. The speed of light is the speed of information. The information "the star has exploded" cannot travel faster than the speed of light, so there is no way to know that a star has gone supernova before that information reaches us. Usually the first particles to reach us from a supernova are actually neutrinos, which can escape from the core of the exploding star a little time before the shock wave of the exploding star reaches the surface and the supernova becomes visible.

It may be possible to forecast a supernova, if (as PM2 ring comments) we could measure the neutrinos from its core before it explodes. But this would be a prediction, not an observation of an explosion. And we can't get that data with the kind of neutrino detectors on Earth.#

It is not possible to know. The speed of light is the speed of information. The information "the star has exploded" cannot travel faster than the speed of light, so there is no way to know that a star has gone supernova before that information reaches us. Usually the first particles to reach us from a supernova are actually neutrinos, which can escape from the core of the exploding star a little time before the shock wave of the exploding star reaches the surface and the supernova becomes visible.

It may be possible to forecast a supernova, if (as PM2 ring comments) we could measure the neutrinos from its core before it explodes. But this would be a prediction, not an observation of an explosion. And we can't get that data with the kind of neutrino detectors on Earth.