To the left of every thread is a little “envelope” indicator that tells me if there are new posts to a thread.
At the bottom of every forum list is a legend explaining the indicators.
“Yellow and open” means new posts.
“Gray and closed” means no new posts.
“Overstuffed” in either color (and open or closed, as appropriate) means more than 150 posts (3 pages).
I have seen numerous threads go longer than three pages, but I have seen no “overstuffed” indicators.
Am I missing something?