They actually can't! They're looking in all directions at once by having 2, and now three detectors on different parts of the planet. So when one thinks it's s

They actually can't! They're looking in all directions at once by having 2, and now three detectors on different parts of the planet. So when one thinks it's seen a GW, it has to correlate the observance with the others, and then they can say "Okay, so given the timing, which way did it come from?" and get a very rough direction. If it's a potential neutron star merger, they can then have everyone swing their telescopes in that direction to hopefully see it happen.