Query Posts based on custom field value

Why not pull the post you need, rather than pull a post and then check the value of the meta key?

$now = strtotime(date('d-m-Y H:i:s')); // get current timestamp

$args = array(
    'post_type' => 'presenter',
    'posts_per_page' => '1',
    'order' => 'DESC',
    'meta_query' => array(
        array(
            'key'     => 'on_air',
            'value'   => $now,
            'compare' => '>=',
        ),
    ),
);