Not sure if I understand right.
Edit:
If page field is limited to one page max you could also do this
if($page->related->related === $page){
// related page has this page also referenced
} else {
// it's not
}
Edit2:
corrected a previous example if multiple page field with better example
if(count($page->related)){
foreach($page->related as $rel){
// check recursively if one of the related pages has this page in it's related pages
if($rel->related->has($page)){
echo "<li>$rel->title</li>";// related page has this page also referenced
} else {
// it's not
}
}
}