Evaluating Form Fields

I have created a contact us form.  I want to evaluate if this Contact assigned_user_name has an assignment.  Most will but if it is a new user, they likely will not have a user assigned yet.  If there is an assigned user, I will just simply send an Alert.  For the ones that do not have an assigned user, I want to send an Alert to a Sales Branch representative. We have about 30 branches.  Is there a way to create branch assignments based on zip code?