New Comment since last Visit

Hi @ all, I have the following scenario: User A views a page with 3 comments, and add's a new comment. Now User B logs in and views the same page. This User already read the first three comments and now I want that User B get's some kind of visual hit(orange triangle or something like that), that a new comment was added since his last visit. How should I aproach this Problem? Any ideas? Thanks a lot.