In 1990 the Galileo spacecraft successfully detected evidence of life on Earth. That wasn’t too big a surprise, but it did make the point that in some cases strong signs of life on a planet can be detected from space by remotely analyzing atmospheric chemistry via spectroscopy.
Spectroscopy means looking at the spectrum of light that’s emitted, reflected, or passing through a substance (including a gas) for bright and dark lines that are characteristic of elements and compounds in that substance. In this case we’re talking about dark absorption lines, wavelengths of light that are absorbed by the atoms or ions in question. If a planet’s atmosphere has a lot of methane together with oxygen, or a lot of nitrous oxide together with ultraviolet light, something must be replenishing the CH4 or N2O that would otherwise not last very long, and based on our knowledge of chemistry, that something is probably some sort of life.
Other than Earth, there are no planets in our solar system with spectroscopic evidence of life, and we can’t quite yet analyze the spectra of light from small, thin-atmosphere, Earthlike planets around other stars, so Earth is so far the only planet where life has been detected that way, and we already knew Earth has life.
But next year NASA hopes the launch the James Webb Space Telescope, which will be large and sensitive enough to potentially detect signs of life. Such life won’t necessarily be complex, let alone intelligent (not that we’re ones to talk), but if it’s discovered, it will of course be a major event. And it might be found in just a few years.