Setting up user verification before adding to the database?

Question

ASP.NET

Good time ...
Help me deal with this problem:
I have authorization and registration with roles and through cookies in my project ... everything is fine with this ...
But I can’t implement it either as checking the user in the database before registering and not a message is displayed stating that the user is not in the database.
Tell me, maybe someone has a simple sketch to see how user verification is implemented