Everytime you log into the site, new posts are marked by the color of the icons. For example, forums and threads which contain new posts since you last logged in will be marked by a gold color icon.
Individual posts use a different icon. Old posts that you may have already read will be marked with a gray icon in the upper left corner next to the date while for new posts, the icon will be a gold color.
There's also a "View Posts Since Your Last Visit
" option at the top right of the forums page after you log in.
I hope that this is helpful and addresses your suggestion.