There are a couple of different methods that can be used to be notified when a topic you have bookmarked has been updated. You can be notified either with a notification bubble above the Bookmarks button in the top menu bar, showing you the amount of bookmarked topics that have been updated since your last visit to the forum, and/or you can receive and email letting you know when certain bookmarked topics have been updated.
To configure your notification options, visit the forum you frequent and login to your forum account. Once logged in, click the Profile button found in the top menu bar, and then click the Edit Profile button found on the next page. On the next screen, click the Notifications tab and you can view all of the different options that you can receive notifications about on the forum. Look for the when a bookmark is updated option, then check one or both of the boxes provided to the left of this area to indicate the type of notification you would like to receive. Once you have your notifications setup the way you want them to be, click the Save Notification Settings button found at the bottom of the page to complete the process.
Once saved, you will begin to receive notifications based on how you set them up. Keep in mind that you will only receive notifications when the bookmarked topics have been updated by other members; you will not receive a notification when you have posted in a bookmarked topic yourself.