There are two methods I use for that.
First, when you post in a forum you automatically are marked as watching the forum. If you choose "Watched Content > By Section > Forum" you can see a list of the forums with new content. The individual topics you have replied to are marked with a star rather than a dot, and the background will be light or dark depending on if you have read the replies.
Second, you can use the profile link under your name menu. Just pick "My Content" from your profile menu and it will show you all the recent threads and their last edit date. All of those should have a star since you replied, but their background will be light or dark depending on if you have read all new replies.