Whether it's Harry Potter, Game of Thrones, or Star Wars, the big franchises all have pretty distinctive and amazing costumes. It can be hard to tell just from watching how a costume was made or what it really looks like, but thankfully there are books and resources out there that give those details!
With some newer fandoms like podcasts or indie games, it can be hard to find official source material. In those cases, see if you can find fan wiki pages. Often they'll have any official info listed, as well as fanart and theories, which can come in handy too.