What's the purpose of $item in taxonomy_field_is_empty($item, $field)?
With the help of @Clive

I think that's not the way, taxonomy_field_is_empty is an implementation of hook_field_is_empty(), which is used to


  Define what constitutes an empty item for a field type.


$item is:


  An item that may or may not be empty.


An item is the data for a specific field entry, so what you'd find in, for example, $node-&gt;field_tags['und'][0] in your code example.

If you have access to the $node variable and your field is named field_tagsyou can do this:

//Recover all the items
$items = field_get_items('node', $node, 'field_tags');
//If there is not a tag, then do something    
if (count($items) == 0) {
  echo "field empty";
}


Or 

//If there is not a tag, then do something
if(count($node-&gt;field_tags[LANGUAGE_NONE]) == 0) {
  echo "field empty";
}