All they need to do is download password dumps (lists of compromised emails and associated passwords) and then use 'credential stuffing' to see which websites those combinations work on. Sadly, it's incredibly easy for hackers to find out whether or not you've been compromised. If your Fortnite account has been hacked it's likely one of your other online accounts was hacked first. It's probably never a good idea to use the same password across multiple sites for one very obvious reason: If one of your accounts is compromised, whoever hacked it can use that same information to hack into the rest of your accounts.
Using the same password across multiple sites is a bad idea.
