They can (in theory) observe them using an observatory named LIGO, or Laser Interferometer Gravitational-Wave Observatory. It's essentially lasers set up to sho

They can (in theory) observe them using an observatory named LIGO, or Laser Interferometer Gravitational-Wave Observatory. It's essentially lasers set up to shoot along long corridors such that any changes in the distance between the ends can be measured. Gravitational waves will change this distance, so the laser interferometer should be able to measure gravitational waves. LIGO hasn't yet observed gravitational waves (it's currently being upgraded to "Advanced LIGO") but has set important upper limits on the strengths of gravitational waves. And importantly, it hasn't ruled them out.