The James Webb Space Telescope, after decades of development, has finally launched. It stands to reveal great knowledge about the dawn of the universe, and great insights into extrasolar planets.
With that thought in mind, I started thinking on what would be needed to not only detect life on an extrasolar planet, but detect intelligent life, that is, some kind of civilization.
I've excluded radio for the purposes of this question, as it is the obvious one - I'm wondering more of if we can see enough about a planet to determine if there is some kind of civilization living on it. With non-intelligent life, we can do things like spectroscopy for detecting elements in an atmosphere. Finding certain molecules in said atmospheres can lead us to hypothesize the existence of life, as we are currently investigating on Venus.
But I'm looking further - to maybe James Webb, but more likely even past LUVOIR. All I can think is being able to see some kind of unnatural light on a planet's surface, and that would probably be difficult for detecting any preindustrial society. For a civilization that uses some form of communication beyond our own however, imaging a planet to find an intelligent species might be the only way.
So, in short, how could we do this? I'm looking for specific types of evidence we could gather, that sort of thing.