How to detect if user has enabled full screen in browser

You can compare the sreen width to the browser width, or height.

if (screen.width == window.innerWidth && screen.height == window.innerHeight) {
    //full web browser
}

EDIT : Be carefull in chrome if user have download manager, translate bar or element inspercter open the height is different to the sreen.

You can use document.mozFullScreen and document.webkitIsFullScreen as below:

if ((!document.mozFullScreen && !document.webkitIsFullScreen)) {
   //FullScreen is disabled
} else {
   //FullScreen is enabled
}

I have done the test on different browsers (IE9, Chrome, FireFox, Opera & Safari) this function works.

function chkfullscreen() {
    if (window.navigator.standalone || (document.fullScreenElement && document.fullScreenElement != =null) || (document.mozFullScreen || document.webkitIsFullScreen) || (!window.screenTop && !window.screenY))
        // full screen
        alert('full screen');
}