The "Detect Displays" feature went away after Lion so that's no longer available from 10.8.0 onwards. However, the same is accomplished in Mountain Lion by simply putting your notebook to sleep, attaching a VGA display and then waking. Upon wake, your notebook will just find the external display. Basically, the same effect as the old "Detect Displays". Of course, initially booting up with the external VGA display attached works as well.
Detect Displays didn't go away, it just hidden, when you are in the Displays Preference Panel press Option Key (or alt key on some keyboards) and the Detect Displays buttons appears, you don't need to put your hackintosh on sleep.