I think a lot of children can do this. I remember in middle school the entire class complaining that the TV was on, with a blank screen, and the teacher having no idea how we knew.

I can still tell when my printer has been left on, though I haven't been a child for a long time. It's actually quite an irritating sound, hence why I leave my printer off most of the time :-P

It has to do with how hearing degrades as a person ages. Some places use this to their advantage and install devices that emit high pitches so as to stop teenagers from loitering there. I know the Ikea in Brooklyn has it, for example.

Problem is, as I aged, all the CRTs disappeared. So which is it?!@!1!?

I'd never thought about that! The whine of TVs used to drive me nuts as a kid, but some time in my late teens, I stopped being able to hear it.

Me too. I went over to see a friend who had an entertainment center with doors that closed over the TV. I walked in and immediately asked if his TV was on. He said it wasn't (the doors were closed and no sound was coming from the speakers). I insisted that it was. To his surprise, it was. They were amazed. I was in my mid-20s.

I still do the same for almost all CRTs. I'm pretty young though. The most annoying was waiting for the ACT to start while hearing a loud super high-pitch electronic whine went on every five seconds.

I too could do this, and even up until very recently I would occasionally hear the electronics.

There also are lasers like that.. fun times in physics class.