WPSE_58094

add_filter( 'the_content', 'wpse_58094_the_content_filter', 20 );
function wpse_58094_the_content_filter( $content )
{
    global $template;
    $templ =  '<strong style="font-size:2em">Current Template Theme File = ' . basename( $template ) . '</strong>';

    if( !is_admin() )
        $content = sprintf( $templ.'%s', $content );

    return $content;
}