###Magento 1.x:
'admin' === Mage::app()->getStore()->getCode()
###Magento 2:
/**
* @param int|string|null|bool|\Magento\Store\Api\Data\StoreInterface $store [optional]
* @return bool
*/
function df_is_admin($store = null) {
/** @var \Magento\Framework\ObjectManagerInterface $om */
$om = \Magento\Framework\App\ObjectManager::getInstance();
/** @var \Magento\Framework\App\State $state */
$state = $om->get('Magento\Framework\App\State');
return 'adminhtml' === $state->getAreaCode();
}