You'll notice the ones in pink have new replies to them; also, you can check the page, in the section at the top "View unread replies to my posts", which will show all threads you've posted in, that have new replies. Also, keeping count of the number of replies, if it's a one-on-one, will tell you if a post has been made without opening it; odd if you started it, even if you didn't, means there's a new reply.