Basically all threads are closed or will be closed or are likely to be closed
Unless someone keeps a thread going, it will be closed - as stated above, after 6 months. However I take your point - as a usability issue it would be nice to know a thread is closed while reading through it, to avoid getting into the frame of mind where you want to post, then find find you can't. It does say 'Closed' at top and bottom, but maybe that's not enough, as it's easy to miss (done it myself).
So, perhaps the answer is a slight color change or something, so that it is more obvious. We might be able to add something like that. Not sure when, though...
...And if anyone can think of an easy option to add in something that tags every post as being part of a closed thread - then let us know, please (I'm assuming there isn't a vBulletin plugin to do that although I could very well be wrong).