How to check if “is single” page

You can use is_singular() with post types like this:

if( is_singular('event') ) {
   // We are in single view of event post type
}

So, your code could be:

add_filter( 'the_title', 'event_page_title' );
function event_page_title( $title ) {

    if ( is_singular( 'event' ) ) {

        $title = "Title: " . $title;
    }

    return $title;

}