Unique Post Meta Values

This should not be complicated so I can’t help but think that I am missing something. What it sounds like you need is get_post_meta.

$yid = get_post_meta($post->ID,'youtube_id',true);
if (empty($yid)) {
  // not set
} else {
  // set
}

Edit:

You need a meta_query for the most WordPress-y solution:

$args = array(
  'post_type' => 'post',
  'meta_query' => array(
    array(
      'key' => 'youtube_id',
      'value' => 'some_value',
    )
  ),
  'ignore_sticky_posts' => true,
  'no_found_rows' => true,
  'posts_per_page' => 1,
  'fields' => 'ids',
);
$q = new WP_Query($args);
var_dump($q);

However, your code is easily adaptable:

$different = $wpdb->get_var("SELECT meta_id FROM {$wpdb->postmeta} WHERE meta_key = 'youtube_id' AND meta_value="some_value"");
if(!empty($different)) {
    echo 'Dude, we already have this post.';
}

Note: use $wpdb. It will save you some effort.