Change the_title() of a page dynamically

I would use the is_page_template() conditional:

if ( is_page_template( 'page-courses.php' ) ) {
    // The current page uses your
    // custom page template;
    // do something
}

Edit

You would use this conditional inside your filter callback:

function wpse83525_filter_the_title( $title ) {
    if ( is_page_template( 'page-courses.php' ) ) {
        return 'Custom Title';
    }
    return $title;
}
add_filter( 'the_title', 'wpse83525_filter_the_title' );

Now, to isolate only the titles of pages that use your page template, you can take advantage of the other parameter passed to the_title: $id. Since you know the Id of the post for which the title is being filtered, you can query the _wp_page_template post meta, and ensure that it equals your page template:

function wpse83525_filter_the_title( $title, $id ) {
    if ( 'page-courses.php' == get_post_meta( $id, '_wp_page_template', true ) ) {
        return 'Custom Title';
    }
    return $title;
}
add_filter( 'the_title', 'wpse83525_filter_the_title', 10, 2 );

Edit 2

If you want to target the “Courses” page specifically, use is_page() with the page slug 'courses', or the page title of 'Courses':

function wpse83525_filter_the_title( $title ) {
    if ( is_page( 'Courses' ) ) {
        return 'Custom Title';
    }
    return $title;
}
add_filter( 'the_title', 'wpse83525_filter_the_title' );

Though, I would recommend changing page-courses.php into a Custom Page Template, which would make this whole process much more robust.