When you click on a discussion with unread posts, it opens at the first unread post.
The "scrubber", which is the timeline navigation on the right of the screen should also show you which part of the discussion is unread.
If you want to find out exactly which post is the first new post, there's visual indicator inside of the post stream itself, but you can also take a look at the URL of a discussion before clicking on it. For example on this forum I see the Fancybox discussion has 1 new post and the URL in discussion list is https://discuss.flarum.org/d/29102-fancybox/36 . So I can click on the date of each post and find out which one is post number 36. Obviously it will be the last one since there's only one new here.
It would totally be possible for an extension to add an "unread from here" line into the post stream like most instant messaging platforms do, but I haven't seen such an extension yet.