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;
}