Notification when a visitor is new (not previously listed in a specific document class)

Good morning,

I have a Value List of names of visitors coming to our organisation.

I’d like to set up an advanced notification to specific persons when the system detects that a visitor has not previously visited our organisation.

In other words, the condition should be: If the visitor is not already listed in a specific document class, then trigger the notification.

How can I get the system to “detect” this — perhaps using a search or calculated property?

Thank you,
Patrick