Steps to prepare:
1) Turn on groupings at site level if not already on. Create grouping in course. Create forum in course. Set forum to separate groups and the selected grouping.
2) Turn on read tracking for that forum. This is INSANELY difficult. Exercise for the reader...
Now when you look at the course page (when logged in as a user who does NOT belong to any groups in that grouping) a notice appears if debugging is enabled:
Notice: Undefined index: 1 in C:\Documents and Settings\sm449\My Documents\apachedocs\corestable\mod\forum\lib.php on line 6065