<?php
$app = JFactory::getApplication();
$menu = $app->getMenu();
if ($menu->getActive() == $menu->getDefault()) {
        echo 'This is the front page';
} else {

        echo 'This is the other page';

}
?>

Refer:

http://docs.joomla.org/How_to_determine_if_the_user_is_viewing_the_front_page