Detect elements which are overflowing your window and causing a side scroll.

findOverflowingElements.js

[].forEach.call(document.querySelectorAll('*'), function(element) {
    if (element.offsetLeft + element.offsetWidth > window.innerWidth) {
        console.log(element);
    }
});

Paste the above into your console or into a javascript file in a section of code that runs after page load. Any element which overflows the window will be output to the console.

As pointed out by @sinan-evanshunt. This can also be solved with CSS

*, *:before, *:after {outline: 1px dotted red !important;}