Get attached media only

All media (somewhat incorrectly) in the $wpdb->posts table will be “attachments” whether actually attached or not. “Attachments” that are actually attached will have a post_parent other than 0, so what you need are all of the attachments that have a 0 in the post_parent column, if I understand you.

$args = array(
  'post_type' => 'attachment',
  'post_status' => 'inherit',
  'posts_per_page' => -1,
  'post_parent__not_in' = array(0)
);
$attachment = new WP_Query($args);

var_dump($attachment->posts);