Suppose life exists on an exoplanet within (say) 100 light-years of Earth. Assume also that such life is not capable of radio transmissions, or anything similar.
What would be necessary to confirm the existence of such life? Might this be possible from Earth-based observation? Would some sort of space mission be required?
Under the right circumstances you can use spectroscopy to get an idea of what an atmosphere is made of, if you’ve got the right equipment and circumstances. There are many volatile compounds that you would be likely to find in an atmosphere of a planet that supported life - things which quickly bond with other elements or otherwise go away and will not remain in an atmosphere, but would be constantly be replenished by biological processes.
At our current level of technology, I don’t think we’d be able to detect life in the universe even if it existed in our solar system. Heck, scientists and others are contemplating the possibility of life in the seas of Europa, but we don’t know whether there is or not, and Europa is a puddle jump from Earth compared to a planet 100 light years away.
Short of sending a rover equipped with a microscope, I don’t think there’s currently any way we can absolutely confirm life on another planet. There are lots of ways we could detect indirect evidence, such as spectroscopically detecting volatile molecules, but I don’t think there’s any case where a geological explanation could be completely ruled out.
We could find planets that are very similar to Earth, in terms of atmospheric composition, and make a case that such a planet is likely to have life. But it would be pretty difficult to detect it directly without actually going there.
Is there any way to detect an enantiomeric excess of some chiral compound by spectroscopy (from a far distance)? That would probably be the best indicator of life (at least of life as we know it).
If we detected significant oxygen in the atmosphere of another planet, that’d be a slam-dunk for it having life. But we could still miss a lot that way, since there could also be life that doesn’t produce elemental oxygen.
Whether we have the technology to do this right now depends on what you mean by “have the technology”. We do not have any equipment currently that could do it, but we know how to build the appropriate equipment, and could do so within a decade or two (which is the typical time frame for designing, building, and launching a satellite mission).
We haven’t really yet figured out for sure whether out next door neighbor, Mars, has any life on it, despite having landed several probes and rovers there. I suppose lots of oxygen in a planetary atmosphere, which we may well one day be able to detect by Earth based observation, would be pretty good positive evidence of life, but it is likely to remain impossible to rule out the possibility of life on exoplanets in the goldilocks zone with any observation made from here, especially when you consider that life there might rely on some other energy source apart from the photosynthesis of CO[sub]2[/sub] into carbon compounds and oxygen.
I doubt if you could even detect that there are lots of chiral compounds on Earth from the Moon. There just isn’t that much of them, and most of them are locked away inside organisms.
And it’s not easy to detect chiral compounds to begin with. The classic test requires backlighting by a known polarized source, which is difficult to arrange on another planet.
One method I’ve heard suggested that I rather liked was looking for city lights on an exoplanet’s dark side.
Obviously, a whole lot of "if"s involved, there.
Just ask them if they’re alive and if they don’t understand, speak louder and slower.
How far away can we detect Chlorophyll?
In visible light it reflects green…and also in the infrared.
I would not count on exoplanetary life using chlorophyll. In fact, I would be rather surprised if it did. There are probably other ways of doing photosynthesis that could have evolved (but that didn’t evolve here because chlorophyll got in first, like the QWERTY keyboard).
…assuming they light their cities with visible light. Instead of ultraviolet. Or sonar. Or neutrinos…
This assumes a planet with clear skies, or at least partially clearing. What about planets with permanent cloud cover? Our planet has a mixture of cloud cover and clearing.
I have always wondered if life forms or intelligent beings on a planet covered in continual haze, like Saturn’s moon Titan, would even be aware of the existence of stars or other planets within their own solar system.
Mankind has looked up at the stars and wondered, as long as we have existed. What about life on worlds where the sky is a featureless bland fog?
I am sure I once read an SF story about a planet with several suns and permanent daylight, where the population went mad every few years when the alignment of the suns allowed them to see the stars.
“Nightfall” by Isaac Asimov.
You’re not the only one!
And in case other readers of this thread have not yet read it:
The book is called “Nightfall” by Isaac Asimov.
And yeah, it’s a great read. A fun story, light and easy to read; but it has enough science combined with emotional reactions that it will stick with you for a long time.
Like I said, a lotta “ifs.” Including that they don’t use fire. 
(Kidding aside, link only tangentially related to the subject of ET lights and cloud-covered exoplanets…but includes “Venusian Emperor” as used in a scientific theory, which makes it all worth it.)
actually, why should we assume that they need artificial light at all?
On earth, lots of animals see very well in the dark. It should be possible to evolve (or breed) animals with two sets of eyes, one for daylight, one for darkness. Or an eye with two retinas, one for daytime, one for nightime.