Wordpress is lacking an is_subscriber method.

gistfile1.txt

function is_subscriber() {
  // Test if role is subscriber
  $cu = wp_get_current_user();
  if (!empty($cu->roles) && is_array($cu->roles)) {
    if(in_array('subscriber', $cu->roles)) {
      return true;
    }
  }
  return false;
}