There are two ways to know if a username is taken on Fortnite. The first one is to try and change your username and before you click submit it will tell you if that username is available or not.
A quicker solution instead of having to load up epic games in your browser and login then try to change your name is to load up a Fortnite tracker website. We recommend this one from here type the username into the search box and click enter.
This will fetch back the stats that username. So, if you get an error message then that username isn’t taken. If it does bring stats back, then you know that that username is in use by someone else.
Sometimes Fortnite tracker will give you an error but you still won’t be able to register that name, this is usually the case if the person that was using that username got banned in which then that username will never be able to be registered again.