From reports that I've read on space based optical interferometers, it will be within our technological capability to make out terrestrial sized planets at distances up to 100 light years or so in continent level detail within the next several decades. If this is true then think of the imaging capability of ET civilizations centuries or more beyond us technologically. Before sending out a message, they could first find a terrestrial planet, then study its atmosphere in detail for gases suggestive of industrial activity. If their systems are good enough, they may even be able to image artificial light sources on the planet's night side. Once they find a world with clear signs of intelligent technological life, they would proceed to send out a signal using the most basic technologies to help increase the odds of it being received.