Officially licensed merch will state so in the listing. No matter how high quality the merch item looks, if it doesn't say it's official, then it probably isn't. This doesn't apply to brands such as Hot Topic, Box Lunch, or the Noble Collection that have a partnership with Warner Bros, and their listings may sometimes skip mentioning the official license. If you're unsure, you can Google the name of the store and see if it's official. But if you find it on a site like Amazon, then it probably isn't official.