Life as we know it requires a planet, so another line of research is to look for evidence of planets round other stars. We can detect these either from the fact that a planet partly obscures the light from its star during a transit (remember the transit of Venus) or from the fact that the star wobbles slightly during the planet's orbit because both planet and star are going round their common centre of mass.

ET life might be intelligent, in which case there might be deliberate or non-deliberate radio frequency out there (e.g. we transmit television shows to the rest of the Universe without intending to as well as the various SETI missions we have launched). So a third line of enquiry can be to look for radio evidence.