Ironically, our first hint of ET will probably be oddly elevated hydrocarbons in an alien atmosphere.
Which hydrocarbons? and why do you think so?
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.