Script that logs and highlights elements that are out of bounds and may be causing unexpected horizontal scroll. Requires jQuery and is in no way optimized.

out-of-bounds.js

/**
 * Sometimes you get horizontal scroll on a page,
 * but can't find the element causing it.
 * 
 * This scrapes the DOM and checks to see if any
 * right edge of an element is beyond the width
 * of a defined container.
 */
(function($){
	// change container to the selector to look within
	var container = 'body';
	
	var content_width = $(container).outerWidth();
	var skip_contents = [];

	// NOT EFFICIENT! Just get the job done.
	var count = 0;
	var found = 0;
	var ignored = 0;
	var out_of_bounds = [];
	$.each($('*', container), function(){
		count++;
		var $this = $(this);
		if (!beingSkipped($this)) {
			// skip elements that won't have scroll
			if ($this.css('overflow') == 'hidden') {
				skip_contents.push(this);
			}

			var $el_right_edge = $this.outerWidth() +  $this.offset().left;
			if ($el_right_edge > content_width) {
				$this.css({
					outline: '4px dotted red',
				});
				found++;
				out_of_bounds.push($this);
			};
		} else {
			ignored++;
		}
	});

	function beingSkipped($element){
		var skip = false;
		$.each(skip_contents, function(k, v){
			if( $element.closest(v).length > 0 ) {
				skip = true;
			}
		});
		return skip;
	}

	console.table([{
		"Scanned Elements": count,
		"Out-of-bounds": found,
		"Skipped Contents": skip_contents.length,
		"Didn't Scan": ignored
	}]);
	console.log("Skipped Children Of: ", skip_contents)
	console.log("Elements out of bounds: ", out_of_bounds);

})(jQuery);

Since this is just a tool for debugging, I just paste in console and run.