In the last 200 years or so we have begun broadcasting radio waves and other electromagnetic waves and they will continue throughout the universe. The same way we are "listening" for sounds of intelligent life, we should expect that they are as well. Consider the fact that they must be more advanced than us in order to travel interstellar distances, then I would assume they are listening as well and would have come to observe us when they received our radio/satellite/tv/internet transmissions.