Is there criteria or a flag added to the invoice process so that if A/P tries to enter an invoice wi

Is there criteria or a flag added to the invoice process so that if A/P tries to enter an invoice with tax, on a PO set-up as non-taxable,  they are alerted?

We placed a non-taxable order to a vendor; the PO was correctly set-up as non-taxable (tax exempt).  When invoice came in against the Purchase Order, the vendor had included sales tax.  The AP data entry user did not catch that the PO was set-up as non-taxable.  It was caught after payment was processed and sent to vendor. Consequently extra work was needed to cancel and re-issue the payment.  How would A/P know if the PO was set to non-taxable? Is there an error message flag that can be set to alert AP that the PO is tax exempt?