Test if a WordPress meta box has been registered.

is-meta-box-registered.php

<?php

/**
 *  Check if a meta box has been registered.
 *  Returns true if the supplied meta box id has been registered. false if not.
 *  Note that this does not check what page type
 *  @param String $meta_box_id An id for the meta box.
 *  @param String $post_type The post type the box is registered to  (optional)
 *  @return mixed Returns boolean or a WP_Error if $wp_meta_boxes is not yet set.
 */
function is_meta_box_registered( $meta_box_id, $post_type = false ){
  global $wp_meta_boxes, $grist_meta_box_found;
	$grist_meta_box_found = false; // assume not found by default
	// if meta boxes are not yet set up, let's issue an error
	if( empty( $wp_meta_boxes ) || ! is_array( $wp_meta_boxes ) )
		return new WP_Error( 'missing-meta-boxes', 'global $wp_meta_boxes is not the expected array' );
	// should we only look at meta boxes for a specific post type?
	if ( $post_type )
		$meta_boxes = $wp_meta_boxes[ $post_type ];
	else
		$meta_boxes = $wp_meta_boxes;
	// step through each meta box registration and check if the supplied id exists
	array_walk_recursive( $meta_boxes, function( $value, $key, $meta ){
		global $grist_meta_box_found;
		if ( $key === 'id' && strtolower( $value ) === strtolower( $meta )  )
			$grist_meta_box_found = true;
	}, $meta_box_id );
	$return = $grist_meta_box_found; // temp store the return value
	unset( $grist_meta_box_found );  // remove var from from global space
	return $return;
}