There are different indicators:
Within a Rule, you have the following info available:
context.stats.loginsCount
-
user.loginsCount
which should be 1 at first signup
Or you also can check the creation date with the login date:
user.last_login
user.created_at
Furthermore, for users of a database connection, you have the post-user registration hook, if you want to trigger certain actions upon signup.
Why do you want to know whether it’s a new user or not? What would you do with this info? Trigger a certain action?