From how far away would you be able to tell that Earth is inhabitanted by intelligent life?
Nowhere but Luna with your eyes, obviously. With an early telescope, it should be pretty from Mars to see the lights, but is there any chance our lights could be interpreted as natural like how Castillo has bright spots?
When you get to Saturn or at least Uranus, you probably wouldn't be able to see the Earth at all with conventional telescopes considering how long it took us to discover Neptune and Naptune should have a far higher angular size than us? From Neptune and beyond would you even be able to tell with a space telescope? Since pictures of the outer dwarf planets don't have any detail, they just look like really bright stars. With modern equipment we would be able to notice the ISS, but how would we tell it is artificial and not just some small captured space debris?