At the top of each board you’ll find Latest, New, Unread, Top. For unread posts, click ‘Unread’. You’ll find all the ones you haven’t seen. After you’ve read them, you can click ‘Dismiss’ which is on the bottom right hand corner of the board. That will clear the posts already viewed.
If you get system notifications you can also see a little blue circle beside the title of each new thread that has new posts in.