Joomla - Check to see if we are in admin area or not

For Joomla 4.0 should be

$app = Joomla\CMS\Factory::getApplication();
if ($app->getName() == 'administrator')   //since   3.2
    echo 'Client is administrator';

It seems to work in Joomla 1.5, Joomla 2.x and 3.x

$app = JFactory::getApplication();
if ($app->isSite())  echo 'Client is site';
if ($app->isAdmin()) echo 'Client is administrator';