When the user enters the forum site, currently there is a bell icon which notifies that there are new posts, and you can see them as a full list only when you press on the notification bell icon. However, it would be better to have a new post notification for individual categories. This information can be placed on the header image of the category so that it reads as:
100 views | 20 posts | 2 new
It will be definitely more user friendly, because it will offer better user experience while surfing among the categories.