I've ended up with following solution:
function _fullscreenEnabled() {
// FF provides nice flag, maybe others will add support for this later on?
if(window['fullScreen'] !== undefined) {
return window.fullScreen;
}
// 5px height margin, just in case (needed by e.g. IE)
var heightMargin = 5;
if($.browser.webkit && /Apple Computer/.test(navigator.vendor)) {
// Safari in full screen mode shows the navigation bar,
// which is 40px
heightMargin = 42;
}
return screen.width == window.innerWidth &&
Math.abs(screen.height - window.innerHeight) < heightMargin;
}
Which works in every browser I care about (Chrome, FF, IE, Safari/Mac, Opera).
Update: It doesn't work on Opera/Mac, Opera on Mac while in full screen mode hides only the 'common' OSX menu, thus height differs only by few pixels which is too dangerous for me.