$args = array(
     // Limits comments to a specific post.
     // Leave this off if you want all comments, blog-wide.
    'post_id' => $post_id,
    // Get only non-approved (ie, pending) comments.
    'status'  => 'hold',
    // Will only fetch comment IDs.
    // If you want full comment objects, leave this off.
    'fields'  => 'ids',
);
$comments = get_comments( $args );