Full screen detection

Does anyone know a smart way to determine weather or not a web App is running full screen or not?
I know I can get screen dimensions for a session, but the problem is that an iPhone 3s or below has the same reported screen
when used in a full screen mode (by using Add to Homescreen) as the iPhone 5 has so checking which device and then screen size alone doesn’t cut it.

It would be very useful if there could be a function that reports if the App is running full screen on any platform, so including Android.
Chrome does a just as good a job of running an App in full screen mode as Safari does on iOS, but I want to detect if users are in full screen mode or not.

Thanks in advance for any replies