But I'm not looking for how to unlock the thread or why the thread was unlocked. My question here is totally different than you are saying. I'm looking for a way to check who locked a thread. Whether it is locked by moderator or locked by the thread starter/OP themself.
You can't find it unless you're the thread creator as mentioned by others. But you can assume by that yourself depends on what kind of thread it is, if it related to a query asked by someone and the answer was found then probably OP itself locked the thread to avoid spamming anymore.