“The world communicates subtly. Most people don’t hear or see the signs because they’re so wrapped up in their day-to-day lives.” -Doug Cooper
If you wanted to see if a planet was inhabited in Star Trek, all you had to do was scan for signs of life. With current technology, that’s really hard to do! We rely on cues from all across the electromagnetic spectrum to identify biosignatures, such as analyzing the atmosphere, land and oceans for molecular signatures.
But we can also look for visual cues of how humans have modified their terrain and even for visual and infrared images of human beings directly. While the Hubble Space Telescope can obtain resolutions of about a foot, an Enterprise-class starship could do 150 times better, seeing individual humans and their motions in real-time if they desired.