How to be safe is important but what happens if you are unsure or just need to check. Also what services your normal username may be linked to? May be something you may be interested to check.
So there is https://usersearch.org which if you enter your username it will check against several online services. It also has some other features as well to explore.
Of greater use is https://haveibeenpwned.com which when you enter your email address it checks against a very large database which contains accounts on various services that have had user details hacked.
If you do find yourself on one of these lists it of course makes sense to change your logon details such as password and any other personel details that may have been changed.