Is 'username' the only way to check for a user's existence? I need to verify that a user doesn't exist through username and email.

Good afternoon Richard,

You can write a custom endpoint to check for a username or an email with relative ease in business logic and return the username or email if found.  You can see some helpful examples of Endpoints in our Business Logic guide, which is available over here:  http://devcenter.kinvey.com/html5/guides/business-logic#Providingatestendpoint

Thanks,

And that can be executed without having an active user?

Richard, 

You can't do anything against the REST Api without some type of authentication being in place.  This would be a great use of an implicit user that doesn't have permission to login, but acts as a service account before logging in.

Thanks,