When you arrive at the Forums, you can click on the "Today's Active Topics" and see posts that were made today -- since midnight. If you visit less than daily, this will only show you the posts today -- not since your last visit.

If you visit multiple times a day, you can still use the "Today's Active Topics" to see all of today's posts. You can determine which threads have new posts since your last visit today by the color of the link.