Javascript + Browser - How to detect when dev tools / firebug is open?

Before anyone of you starts saying it's impossible, please navigate to facebook.com open the devtools, even in non attached mode, and it will tell you that dev tools is opened, right?

So how is facebook able to pull that off?

I am able to do it for attached windows

window.outerHeight - window.innerHeight > 200 || window.outerWidth - window.innerWidth > 200

But when the window is popped up, this doesn't work.

How is facebook able to pull this off?