using the watch list would only be good if you oath wise want to watch it. However if you want to casually monitor and potentially reply to it then this would not be the best solution.
Why not? You click watchlist, you see someone has posted a reply (the casually monitor part) and can decide if you want to read it now (click on it) or not (do not click on it). If you read the new replys you can still decide if you want to answer or not. And even if no new replies came in since you last checked and now want to answer you can still find the thread under Watchlist -> Edit Watchlist.
I dont see how going to the subboard in question and searching the x first pages for the thread is better.
Also OP asked about topics started, which are automatically on your watchlist and have to be manually removed.