MC4WP_API_v3::get_list_member PHP Method

    public function get_list_member($list_id, $email_address, array $args = array())
    {
        $subscriber_hash = $this->get_subscriber_hash($email_address);
        $resource = sprintf('/lists/%s/members/%s', $list_id, $subscriber_hash);
        $data = $this->client->get($resource, $args);
        return $data;
    }

 /**
  * Checks if an email address is on a given list with status "subscribed"
  *
  * @param string $list_id
  * @param string $email_address
  *
  * @return boolean
  */
 public function list_has_subscriber($list_id, $email_address)
 {
     $data = $this->api->get_list_member($list_id, $email_address);
     return !empty($data->id) && $data->status === 'subscribed';
 }