When you look at the "bell" icon in the upper right corner, it shows recent posts in threads that you are have posted in/subscribed to (although it always seems to be a bit erratic - sometimes I get 12 alerts, sometimes I'll check in after a day or two and there won't be any alerts at all).
Under the old format, newer alerts that I hadn't already looked at had a little yellow asterisk to indicate that they were unread.
Under the new format, that doesn't seem to happen anymore and so it's more difficult to tell if you've already looked at a post/thread or not.
Under the new format, that doesn't seem to happen anymore and so it's more difficult to tell if you've already looked at a post/thread or not.

if you want to alter the way alerts are presented you can go to your account page and preferences to see the settings for alerts:

checking alert page skips mark read should stop the annoying default behavior i mentioned above.