If we're going to find an extraterrestrial intelligence (ETI), we're going to recognize their signatures. These signs can be organized into three basic types: spectral signatures, physical artifacts and structures, and remnants, or byproducts, of their technological activities. Some of these signatures could be deliberate attempts to get our attention, while others are simply leakage.

Should we detect these signatures, however, the challenge will in determining which are the work of an ETI and which are the products of some unknown or bizarre natural phenomena (like the time we thought quasars were an alien beacon).

14 Intriguing Ways We Could Detect Signs Of An Alien Civilization