The earth detecor telescopes being prepared for launch in a couple years will look for a signature that matches, within a range, the signature of earth. The comparison signature of earth has already been obtained by deep space satellite. The main elements of the signature are oxygen atmosphere and some water vapor. The oxygen atmosphere alone would be a strong indicator.
But there's much more to our tenuous existence than just that. Without: the tilt of the earth, the moon in orbit, the rotation of the earth, the atmosphere sheilding off solar radiation, etc., climactic conditions would be so harsh as to prohibit human life. If that series of comet impacts that hit Jupiter a few years back, had instead hit earth, there would be no life on earth of any higher order. The clouds of dust from the impact were larger than the earth. Jupiter
It is an awesome and wonderous site to view the works of the almighty God of the universe. It is humbling to meditate, by comparison, on the frail, yet beautiful aspect of our individual existence.