At night, you can see millions of megawatts of electric lighting, which, however could be interpreted as naturally-occurring oil well fires. There has been a persistent tale that astronauts could see the great wall of China, but this is impossible because the wall is only 50 feet wide or so; nothing short of a good-sized telescope could see such fine resolution clearly from orbit.
The bottom line is that, casual inspection from orbit shows no discernible trace of artificial structures. At radio frequencies, however, the Earth comes alive in a complex and powerful song of radio beacons, music stations and other non-natural emissions. Earth out shines the sun itself in many narrow bands of the radio spectrum and would be detectable from stars 50 light years away.