Make Customer PO Required when entering Sales Orders

I'm trying to find a simple way to make the customer's PO number required when entering sales orders.  This is extremely common for us where some customers require POs on their orders and some do not.  Past systems have always had a flag on the customer record as to whether the PO is required on the sales order.  This seems like such a simple and fairly ubiquitous business requirement, but I cannot find anything in the system to allow this.  The requirement should be enforced regardless of whether the user is placing the order online, or one of our customer