It is not just gravitational waves, they look at light, with telescopes (visible, infra and ultra), spectrographs, interferometers, radio signals, gamma rays...

It is not just gravitational waves, they look at light, with telescopes (visible, infra and ultra), spectrographs, interferometers, radio signals, gamma rays... Then they combine all that info to come up with cohesive theories. LIGO just by itself would be almost useless.