Laravel select where exists query

I'm trying to create a query in laravel which will check if a username exists in the record where email equals $email. My table is called User.

This is because when a user registers, they only register with their email address and password and add a username later, so this is to check if they have added a username yet.

I have checked the docs here: http://ift.tt/2mPTBqR and can't understand which query I need to use, can anyone help?

via Chebli Mohamed