User::isExternalUser

static function isExternalUser($userID)
{
	static $result = [];

	if (array_key_exists($userID, $result))
	{
		return $result[$userID];
	}

	if (!ModuleManager::isModuleInstalled('extranet'))
	{
		$result[$userID] = false;

		return $result[$userID];
	}

	$dbResult = CUser::getList(
		'ID',
		'ASC',
		['ID_EQUAL_EXACT' => $userID],
		['FIELDS' => ['ID'], 'SELECT' => ['UF_DEPARTMENT']]
	);

	$user = $dbResult->Fetch();

	$result[$userID] = !(is_array($user) &&
						 isset($user['UF_DEPARTMENT']) &&
						 isset($user['UF_DEPARTMENT'][0]) &&
						 $user['UF_DEPARTMENT'][0] > 0);

	return $result[$userID];
}