I almost always click on the View New Content link rather than scrolling though the messages on the main page.
If you do that, too, and wonder what the icons mean, here you go. I know that they look sort of similar
The little quote bubble on the right side means that you've read the post and already responded.
The circle with the highlight in the center means you haven't read it yet.
If there's no icon at all, you read it and moved on.