This small jQuery function checks if elements that are positioned as sticky (position: sticky;) are, in fact, in their ‘stuck’ position. It’s a very simple check where the script checks if the element’s position is equal to its ‘top’ CSS variable. If it is, it adds the class you specify (argument className), and if not, removes it.

hnl.checkStuck.js

$.fn.checkStuck = function (className) {
    $(this).each(function() {
        var t = $(this); //preselect
        t.toggleClass(className, (parseInt(t.css('top'), 10) === t[0].getBoundingClientRect().top));
    });
}