I don't see a way either.So... there's no obvious (to me) way to mark specific forums as "read".
EDIT: Xyro posted on the next page you can mark threads read on the New Posts page. Link at top of site

You have to view each thread for it to be marked read. If you come back to the site in 30 minutes threads you haven't read will still be unread.Please don't tell me that there's no way to do this, and that Xenforo considers everything "read" every time I visit the site.