Detect parent nav items and adds a class of "parent-menu-item" using Walker_Nav_Menu.

gistfile1.php

<?php
/**
 * Updates the walker arg with custom class for detecting parent nav elements.
 */
function cc_add_menu_walker_arg( $args ) {
 
    $args['walker'] = new CC_Detect_Parents();
 
    return $args;
}
add_filter( 'wp_nav_menu_args', 'cc_add_menu_walker_arg' );
 
/**
 * Detects menu items with children and adds class of parent-menu-item.
 .
 */
class CC_Detect_Parents extends Walker_Nav_Menu {
 
    function display_element( $element, &$children_elements, $max_depth, $depth=0, $args, &$output ) {
 
        $id_field = $this->db_fields['id'];
 
        if ( is_object( $args[0] ) )
            $args[0]->has_children = ! empty( $children_elements[$element->$id_field] );
 
        return parent::display_element( $element, $children_elements, $max_depth, $depth, $args, $output );
    }
 
    function start_el( &$output, $item, $depth, $args ) {
 
        if ( $args->has_children )
          $item->classes[] = 'parent-menu-item';
 
        parent::start_el(&$output, $item, $depth, $args);
    }
 
}
?>

gistfile2.css

.parent-menu-item > a::after {
        content: '+';
        padding-left: 20px;
        padding-left: 2rem;
}