How to read user profile by email address on custom login page even before the user logs in

We are bulk migrating our users from our external database to Auth0 using Auth0 import/export functionality. We are moving them from a 3rd party vendor database and we don’t know their passwords. We don’t want to send them a reset password email. Instead, when they login first time, we want to show them a customized error message saying they have to click on forgot password and reset their password. But the problem is without reading the user profile how can I find out if it is the user’s first login. So my question is how to read user profile by email address from the login page even before he logs in.