Hi Janice,
An alert can only be triggered by a query, and there is no way to query whether a user is printing a document, or trying to do so.
You could check whether an added document has been printed, which would defeat the purpose.
Either way, it would rely on a supervising user seeing the alert, and this person being able to intervene in time. Such a process cannot be guaranteed to work.
The only other simple / cheap solution is office policy and user training ("only ever print documents that have actually been added" and "always look at the button, it must say 'OK' ").
Certain documents can be set to print automatically (deliveries and invoices for example), upon adding, in which case you would make it easier for the user not to make this mistake.
Regards,
Johan