$menu = $app->getMenu();
if ($menu->getActive() == $menu->getDefault()) {
 echo 'This is the front page';
}

or use the following for multiligual sites

$menu = $app->getMenu();
$lang = JFactory::getLanguage();
if ($menu->getActive() == $menu->getDefault($lang->getTag())) {
	echo 'This is the front page';
}
else {
	echo 'Accueil';
}

Note that $app is predefined in root/index.php