You can see how many have been posted in the rooms you have open but are not in and they have a red band when you have been mentioned. There isn't a count for when you have been offline because it is a real time chat room unlike facebook.
I meant on tab, before 'Coinchat' name.
~~MZ~~
They already do that, though only for messages directed to you,
for example when they address me, or PM me,
the amount of messages show up in the tab