Here, let me show you show you something:
See those topics with faded text? Those are topics I’ve read every post in.
See the pinned topic has a black title but no unread count and no new dot either? That’s because I’ve never read that topic while logged in and it very much predates my account creation date. 
So you don’t need to do anything to tell which topics that predate your account you haven’t read. They’ll become the faded version when you’ve read all the posts in them same as any other topic would.
Internally, until there is an entry in the database saying you’ve seen the last post in a thread, it will always be in the unseen state even if it predates your account creation, it just won’t be marked as Unread until there is an entry that says you have read the topic but there are still posts in it you haven’t read.