wordpress-populate-column.php

<?php
/**
* For each page each, loop through registered columns, and output value
* @param String - current column name
* @param Int - current page ID
*/
function populate_column ( $column, $page_id ) 
{
    switch ( $column ) {
        // if the current column name is template
        case 'template' :
            $p = get_page_template_slug( $page_id ); // get current page template 
            echo ($p == '' ? 'Default Template' : $p); // write 'Default Template' if no custom template detected
        break;
    }
}
// hook our function nito Wordpress
add_action('manage_pages_custom_column', 'populate_column', 10, 2);