Wheniwasyourage
If you aren't on a deleted thread yourself, how do you find out that that is what has happened?
There's not really a way of knowing a thread has been deleted - unless you have the URL, in which case the deleted message/reason will appear when you try to load the page. This will also happen if you've posted on the thread, you can see the message under the title on 'I'm On'.
Alternatively, you could click 'watch this thread' (in a green box on the right, just above the OP) to watch any threads you don't want to post on. To find the threads, click on 'Watching', next to 'I'm On'.