If at the same time* we were to observe a supernova via other telescopes, that would give a pretty good indication. :)
*: And this would also possibly serve to answer another question -- do gravitational waves travel at precisely the speed of light? (also, I'd be interested in hearing about neutrino detectors -- how closely (if nonzero) to the speed of light do neutrinos travel?)
Yes, gravity waves travel at lightspeed, at least if you buy into general relativity's tensor equations.
Neutrinos have rest mass and move slower than light. We know this because neutrinos change their flavor in transit, meaning they experience the passage of time, something that would not be possible if they had no rest mass and traveled at light speed.
Nobody knows how much slower, but they must be moving almost light speed because we see neutrinos from supernova collapse before we see the light emission. (The light emission from supernovae is delayed by several hours, because it takes that long to heat up the gas around the collapsed core before it can radiate out.)
*: And this would also possibly serve to answer another question -- do gravitational waves travel at precisely the speed of light? (also, I'd be interested in hearing about neutrino detectors -- how closely (if nonzero) to the speed of light do neutrinos travel?)