An entity reference field, by definition, can only target one entity type so you can know what your target type is by calling up [`getSetting()`][1] on the field definition.

    // Print the targeted entity type field.
    $field = \Drupal\field\Entity\FieldStorageConfig::loadByName('node','field_tag');
    echo $field->getSetting('target_type');

Or better yet, use [`EntityReferenceFieldItemList::referencedEntities()`][2] to pull up the node's referenced entities and the associated data you need.

    // Return an array of Entity objects referenced in the field.
    $node->field_tag->referencedEntities();

  [1]: https://api.drupal.org/api/drupal/core%21modules%21field%21src%21Entity%21FieldStorageConfig.php/function/FieldStorageConfig%3A%3AgetSettings/8.2.x
  [2]: https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Field!EntityReferenceFieldItemList.php/function/EntityReferenceFieldItemList%3A%3AreferencedEntities/8.2.x