extended is_page() please?

For now, you can find if a Page is the child_of a particular Page:

<?php
if (3 == $post->post_parent) {
// do something
}
?>

That must be used inside The Loop

or
<?php
if (3 == $wp_query->post->post_parent) {
// do something
}
?>

That can be used anywhere (inside or outside The Loop).

A more generic descendent_of is harder.