Display the name of the current WordPress template being used

functions.php

<?php
/**
 * Debug function to show the name of the current template being used
 */
function mytheme_show_template() {
	global $template;
	if ( is_user_logged_in() ) {
		echo '<div style="background-color:#000;color:#fff;z-index:9999;position:absolute;top:0;">';
		print_r( $template );
		if ( is_single() ) {
			echo ' (and using Post Format: ' . ucfirst( get_post_format() ? get_post_format() : 'standard' ) . ')';
		}
		echo '</div>';
	}
}
add_action( 'wp_head', 'mytheme_show_template' );