A thread on another forum got me thinking, you know in Star Trek they're somehow usually able to scan a ship or a planet for lifeforms? I wonder, what way could you realistically scan for a life remotely like that?
Spectroscopy looks like a good approach - if there are unstable combinations of gases in the atmosphere (like methane plus oxygen at the same time), something had to produce them recently.
Technology could reveal itself via the emission of significant light at night, intense radio waves or similar things.