Neutrinos are generated in high civilization starts building megastructures which block off a large portion of their star’s light, we should be able to detect evidence through our search for extrasolar planets.
A gigantic space station would give off a much different light signature than a nice spherical planet as it passes in front of its star.
There have been a few attempts to reach out to other worlds directly, transmitting signals out into space. It’s unlikely that these signals will actually reach any other civilization, and some scientists are concerned about the wisdom of this kind of communication.
Do we really want to alert potentially hostile aliens to our location in the Fraser Cain - Universe Today