Couldn't it be added in the mainpage, on the most recent forum topics, a line saying to which forum a given post belongs to?
For example...

Simple but fun and addictive.... (replies, author, etc... as it is)
(Off Topic) <--- in a bit smaller fonts maybe...
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What happened Boosted???? (replies, etc, etc)
(Seventeen Or Bust)


As to one know about what the post is, sometimes it's not understandable just by the subject line.