At Big Think, astrophysicist Ray Jayawardhana talks about how to know a supernova is there even if you can't see it.
The answer is neutrinos, the little weird particles that are so small they usually zip around the universe without hitting anything. To find them you need very large detectors that are also sensitive enough to know when a neutrino has hit them. Supernovae, which are massive explosions that happen when massive stars finally run out of juice to keep burning, tend to fire off massive amounts of neutrinos -- they emit as much energy in the explosion as the sun will during its entire lifetime (something around 10 billion years, give or take). So when a detector senses such a surge, it knows something's happened.
As Dr. Jayawardhana points out, the neutrinos will reach the detector even if there's a large gas cloud or some other phenomenon between us and their source. So we would know a supernova might have happened without seeing it.
What struck me was what he said was the kind of kickoff event for neutrino astronomy, back in 1987. Three different detectors showed a result that indicated some kind of massive explosion in a nearby dwarf galaxy. The "surge" that told scientists the explosion happened and where?
A whopping two dozen neutrinos. My guess is if they ever detect three dozen, it'll be time to get right with your Maker, 'cause whatever pumped out that kind of surge is going to play merry Hobb with the old life-continuing-on-the-planet thing.