Etikett: is_page

global $post;     // if outside the loop

if ( is_page() && $post->post_parent ) {
    // This is a subpage

} else {
    // This is not a subpage
}

Eller

functions.php

function is_tree($pid) {      // $pid = The ID of the page we're looking for pages underneath
    global $post;         // load details about this page
    $anc = get_post_ancestors( $post->ID );
    foreach($anc as $ancestor) {
        if(is_page() && $ancestor == $pid) {
            return true;
        }
    }
    if(is_page() && (is_page($pid))) 
               return true;   // we're at the page or at a sub page
    else 
               return false;  // we're elsewhere
};

if(is_tree('2')){ // 2 being the parent page id
   // Do something if the parent page of the current page has the id of two
}