[Wordpress] How to check if page is a child?

Paste this into your functions.php:

function is_child($pageid) { 
    global $post; 
    if( is_page() && ($post->post_parent == $pageid) ) {
       return true; // is child
    } else { 
       return false; // not child
    }
}

In your template use if like this:

if (is_child(X)) {
    //do somthing
}