In the end, a million pieces of hardware scattered throughout the solar system will do a lot for us and the capability is mostly in place while the economics are not too much behind.
Science Fiction almost always imagined interstellar travel first then discoveries of alien planets upon arrival, and lately I have heard SF authors jokingly complaining that exoplanet discoveries make their job more difficult.
Personally I can recommend also the fascinating novel Blind Lake by Robert Charles Wilson which touches on future imagined telescope and consequences of such discovery.
I have also stumbled upon an amusing concept called Galactic Life Imager on he net, however I don’t think we will see it soon ;)
Of course this also means that if they exist most likely they know of our existence-leaving the question-why don’t they want contact? Personally I believe that due to time difference, as millions of years of evolution both technological and biological will make them unable to communicate with us or at least without us losing our unique culture and development.
Of course there is also the possibility that they simply faded away or never existed at all.
Still, we should have tools to provide some answers soon-at least within next 50 years.
Approximate world-wide use of power: P = 2 * 10^13 W
Number of photons generated by city lights (per planet): N = P/e ~ 6.6 * 10^31 s^-1 (per second)
Area of sphere of 100 ly radius: A = 4 pi r^2 ~ 1.2 * 10^37 m^2 (square meters)
Number of photons impinging on telescope aperture: N/A ~ 5*10^-6 s^-1 m^-2 (photons per second per square meter)