Find elements whose width is greater than window.innerWidth, possibly resulting in appearance of horizontal scroll.

find-horizontal-scroll.js

// Throw this into dev console and set 'max-width: 100%' for returned list of elements
Array.from(document.querySelectorAll('*')).filter(el => {
    if (el.getBoundingClientRect().width > window.innerWidth) return true;
    
    const {
      width,
      paddingLeft,
      paddingRight,
      marginLeft,
      marginRight
    } = getComputedStyle(el);

    return [
      width,
      paddingLeft,
      paddingRight,
      marginLeft,
      marginRight
    ].reduce((sum, val) => sum + parseInt(val), 0) > window.innerWidth;
});