How to Access Global $multipage or Global $numpages outside the loop?

I am writing a code in the header that pertains to link rel=”prev” and link rel=”next” thing but I need to check if the post is multipage. I check out these global variables:

$numpages
$multipage

However they don’t work in header.php or outside the loop? Example code to verify in header (just a test, but it does not work):

<?php
global $wp_query;
global $numpages;
global $multipage;
If ($multipage) {
echo 'This is a multipage post';
} else {
echo 'This is not multipage';
}
?>

I am wondering how to do this, can somebody help me please? Thanks.

Just inspect the current post content for '<!--nextpage-->':

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 );
}

On page 2 of 3 that returns:

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

On an unpaged post it returns:

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