IceCube neutrino observatory, and noted that they don’t detect neutrinos directly, but instead detect flashes of light that occur when a neutrino strikes a water molecule. What I didn’t say is that the flash of light is due to Cherenkov radiation. The interstellar neutrinos have so much energy that when they strike an oxygen nucleus a muon (one of the electron’s heavier cousins) is emitted with a speed faster than light in water or ice. The muon creates Cherenkov radiation which can be observed by the photodetectors, as seen in the figure above.
Since the Cherenkov radiation is a directional cone, by timing the arrival of the flash at different detectors in the array we can determine not only the energy of the neutrino, but the direction of its origin. This is how we can determine that they really are from interstellar space, and not due to something like cosmic rays hitting the atmosphere.