function wpsnipp_show_all_custom_fields() {
    if ( isset( $_GET['post'] ) ) {
        $post_id = absint( $_GET['post'] );
        ?>
        <div id="message" class="updated">
            <h3>All post meta:</h3>
            <xmp><?php print_r( get_post_meta( $post_id ) ); ?></xmp>
        </div>
        <?php
    }
}
add_action( 'all_admin_notices', 'wpsnipp_show_all_custom_fields' );