An even easier thing to do is to check the stickies, and then check the first few pages of whatever sub-forum you're in. While the Thread Search GhostOfMatrix linked to is useful, it will bring up every thread in the history of ever.
So you can make a thread on a topic thats already been made before, just make sure there arent any active threads with the same topic.
IF you make a similar thread to one thats active, it would be locked because it was a duplicate.
I don't like the google search thing... But if you can't find the topic it is so old that the most people don't remember it, but it is a good suggestion for AG3.
Also, what Maverick said is correct. Looking through the stickies in each sub-forum is always a great thing for general information.
But if you can't find the topic it is so old that the most people don't remember it
Most of the popular topics are on pages one-ten of each sub-forum. However, I suppose people don't have the patience to look through those pages to find the more popular topic rather than creating a duplicate thread. If the thread is about a year or two old, then it's best to just create a new thread for that topic. Unless you have a great post for that old thread that really contributes to it.