In the main, I think we'd simply have no chance of detecting such civilisations. In the next couple of decades, however, advanced space telescopes with starshad

In the main, I think we'd simply have no chance of detecting such civilisations. In the next couple of decades, however, advanced space telescopes with starshade coronagraphs could be able to start doing spectroscopy on exoplanet atmospheres.[1] This would definitely reveal (some kinds of) biologically-active atmospheres, although it wouldn't necessarily to be able to tell the difference between a complex biosphere with a low-tech civilisation, and a uni-cellular algae soup. I suppose there's an outside chance that such a telescope could detect combustion compounds that are difficult to explain geologically or biologically, and that might be used to hypothesise a civilisation. Something like lead smelting might leave a pretty distinct signature, for example. But that would just lead to a hypothesis, not proof, and I imagine it'd be pretty contentious. Longer-term, it should be possible to create arbitrarily large space-based optical interferometers[2], to the point where one would be capable of imaging cities and agricultural activity on exoplanets. That'd settle matters (Unless it's under water or impenetrable cloud decks, and then you're just out of luck). But that level of resolution is probably a couple of centuries and/or one singularity away from happening, so don't hold your breath. 1: http://www.planetary.org/blogs/guest-blogs/2017/20170623-starshade-helping-space-telescopes.html http://www.planetary.org/blogs/guest-blogs/2017/20170623-sta... 2: https://www.noao.edu/meetings/interferometry/workshop-files/Carpenter-Space-comp.pdf https://www.noao.edu/meetings/interferometry/workshop-files/...