Return the class of the field's label.
public function elementLabelClasses($row_index = NULL) {
$classes = explode(' ', $this->options['element_label_class']);
foreach ($classes as &$class) {
$class = $this
->tokenizeValue($class, $row_index);
$class = drupal_clean_css_identifier($class);
}
return implode(' ', $classes);
}