Along the Forums bar (see picture) there is a What's New option, which will display all of the threads in the forums that are new or have responses since you last viewed them. The only thing is, on the old forum, when you closed out your window or were inactive for a period, all of the threads would then show as "read". With the new forum, once you've caught up with all the threads you are interested in, you'll want to hit the "Mark Forums Read" button which is on the top left corner under the Forums tab (you can also see it in the image below).
The benefit here is if you have to stop reading threads before you're caught up, you get to control the "reset" of the forums. Helpful if you only have a few minutes at at time to catch up, or if you get sidetracked while viewing the forums, etc.
If you go into each forum separately, you'll also see those threads that have new activity as bold, and those that you've read or from the last time you visited and hit Mark Forums Read will not be bold text.
Not the same as the method you were using to see when you were last on the forums, but this way will work the same if you remember to use the "Mark Forums Read" button when you are caught up each time, as then only new material will show up in the What's New section or as bold text in each forum.
Hope this helps!