You can either use manual mark or last visit cookie to keep track of new messages.
In your user preference, if you select yes for Use MARK time stamp feature?, you will have
to manually mark (by clicking on at the topics page)
a forum you just read. This will timestamp the forum so the next time you return, only the topics containing messages posted
after the mark timestamp will be indicated by the color icons. If you choose not to use the mark option,
new messages will be indicated according to the time of your last visit to the forum.