Creating a Query for Purchase Order Approval

I need to develop a query to check if any of the line totals in a new purchase order exceeds $500. I have to attach this query to the approval procedure (under 'Terms' in 'Define Approval templates'). When the 'Add' button on the purchase order is clicked, the values on the form are still not available in the database. This means, my query has to pick the data from the new purchase order form. Is there any way to build such a query? Thanks.