You have different options for marking posts.
Every group or forum has a tab on it's main page that says "Mark as Read"
This will change the color of boxes at the beginning of the from yellow to white.
Go to any thread and you will see two options at the bottom of the main post.
Always highlight: 10 newest replies | Replies posted after I mark a forum
After you mark a group or forum, threads will go from white to yellow based on your choice.
If you choose "10 newest replies", the next time you open the thread, only those will be yellow.
If you choose "Replies posted after I mark a forum", all new replies will change to yellow.
It sounds like you would like the second option. A thread would be yellow if it had any new replies after you last read it.
The group or forum will also change to yellow, indicating there are new responses.
I hope this helps.