I think the only way you can check if a username is available is when you are creating a new account.
When it says "choose your username" it has a textbox, when you write something and click continue, 2 things can happen:
1. Username is not available and throws an error.
2. Username is available and you automatically get it (no "confirm yes/no" question)
So, if the second thing happens, the username won't be available anymore and it would mean creating a new account everytime you get a working username.