How to add drop symbol to parent nav items which have child

Each WP menu item that contains a submenu haves the “menu-item-has-children” class for <li> you can use that class for add a pointer for sub menu:

For example the simple example can be this but you can use that for creating better pointer for your site:

.menu > li.menu-item-has-children > a:after{
   content: "(have child)";
}