If you look at the Earth from space, it's pretty clear that there's intelligent life here because you can pick out cities on the night side. Abraham Loeb, of Harvard University and Edwin Turner, from Princeton University, are saying that we might find other civilizations the same way.
The reason is that the way we usually listen for aliens - via radio signals - may not work that well. One issue is that contrary to popular belief, TV transmissions don't travel all that far. (It would actually be near-impossible to pick up old shows from more than a few light years away).
Our own civilization is actually generating less residual energy as time goes on as communications move to optical fiber. Other civilizations may have done the same, so picking them up actually gets harder as their technology gets better.
The trick is to look for a certain kind of light that would be different from natural starlight. The two note the spectrum of LEDs and streetlights is different from that of the sun. An alien looking at Earth with a sufficiently sensitive telescope would notice that.