How to find if you are on the last page of a multi-page post in WordPress. Explanation at http://sudarmuthu.com/blog/how-to-find-if-you-are-on-the-last-page-of-a-multi-page-post-in-wordpress/

last-page.php

<?php
/**
 * Is the current page the last page of the multi-page post.
 *
 * @return bool True if last page, False otherwise.
 */
function is_last_page() {
	global $page, $numpages, $multipage;

	if ( $multipage ) {
		if ( $page != $numpages ) {
			return false;
		} else {
			return true;
		}
	}

	return true;
}