window.onload = function() {
    for (i=0; i<form.elements.length-1; i++) {
        elem = document.forms['quick_reply'].elements[i];
        elem.focused = false;
        elem.hasFocus = function() {
            return this.focused;
        };
        elem.onfocus=function() {
            this.focused=true;
        };
        elem.onblur=function() {
            this.focused=false;
        };
    }
    }

http://www.ozzu.com/programming-forum/using-javascript-check-the-focus-element-t76858.html

check if <input> or <textarea>  has the focus

JavaScript