two functions to: extract a posts media items, and extract all library items. can use to compare

WP-media-and-library-items.php

<?php

function myplugin_find_media($content) {

    $mediaRegex = "/(src|href)=\"(.+?).(jpg|png|pdf|gif)\"/i"; // your support filetypes
    $mediaFind = preg_match_all($mediaRegex, $content, $media);

    if (isset($media[2]) && count($media[2]) > 0)
        return $media[2];

    return false;
}

function get_library_items() {
  // get the media library for comparison
  $library = array();
  $args = array(      
      'post_type' => 'attachment',
      //'post_mime_type' => 'image', // if theres only one
      'numberposts' => -1,
      'post_status' => null,
      'post_parent' => null,
  );
  $attachments = get_posts($args);
  foreach ($attachments as $post)
      $library[$post->ID] = wp_get_attachment_url($post->ID); // or add more info about thumbnails, etc
}