We don’t have an update notification feature at this point in time.
Some authors maintain mailing lists for update notifications, or give them via Twitter or Facebook. If so, it’s generally stated in the story by the author.
Alternatively, you can add a story to your Currently Reading section on your archive profile. Our FAQ here explains how to do that. It gives you a quick link to a story, and you can see the last time it was updated on that page as well. You can also see if the author has a “Promote a Story” thread, and follow that thread, with email notifications enabled.