<?php
/**
 * Determines whether or not the current post is a paginated post.
 * @return   boolean    True if the post is paginated; false, otherwise.
 */
function acme_is_paginated_post() {
    global $multipage;
    return 0 !== $multipage;
}
?>

<?php if ( acme_is_paginated_post() ) { ?>
<div class="post-pagination-toc">
        <!-- paginated post TOC  display that checks headers across different poages and the displays all headers in as TOC's content uniformly across all paginated pages -->
</div>
<?php else { ?>
<div class="normal-singlepage-toc">
    <!-- normal TOC display for single non-paginated post -->
</div>
<?php } ?>

<?php
function TOC_new () {
global $numpages;
if ( is_singular() && $numpages > 1 ) {
    // This is a single post
    // and has more than one page;
    // Implement a General TOC to be displayed consistently multiple Pages and return values
} else {
    // This is a single post
    // and has only one page;
    // implement TOC code for single pages.
}
?>

function wpse_check_multi_page()
{
    $num_pages    = substr_count(
        $GLOBALS['post']->post_content,
        '<!--nextpage-->'
    ) + 1;
    $current_page = get_query_var( 'page' );
    return array ( $num_pages, $current_page );
}

Array
(
    [0] => 3
    [1] => 2
)

Array
(
    [0] => 1
    [1] => 0
)