There is no update notifier for Xfce4. An update notifier is an icon that stays in taskbar (notification area) and which warns the user when updates are available. The user then just has to click on the icon to see the list of available packages to update and click "update" or "cancel". Gnome or Kde provides one. I've marked this bug as "major" because this leads users to not update their system and thus not get security fixes ! Originally I've submitted a bug in Fedora bugzilla, but I was told to submit it here : https://bugzilla.redhat.com/show_bug.cgi?id=1073393
There is no general tool to watch for new updates, too distro specific.