How to use Greek characters/letters in a query?

This is the loop I’ve created to display post’s matching criteria on empty page:

// get results
$the_query = new WP_Query( 'meta_key=apaitei_logariasmo_facebook&meta_value=Ναι' );

// The Loop
if( $the_query->have_posts() )
    while ( $the_query->have_posts() ) {
        $the_query->the_post();
        echo '<h3>'; the_title(); echo '</h3>';
        the_content();
    }
wp_reset_postdata();

It works. Look at the image:

Note: my site’s language is English with:

define('DB_CHARSET', 'utf8');
define('DB_COLLATE', '');

How do you know that your query returns no posts if you don’t display anything. At least the code you provided does not show it.