Interrogate PO/Receipt Before Bill

I don’t think I’ve seen this in the Community so far.  My customer needs to insure received items are properly QC’d in-house before paying the vendor bill (medical equipment). 

My idea is to place a custom field (check box) or date on the item receipt called “QC Complete”, then deploy a User Event script to interrogate the field and alert users when billing POs if a receipt has not been QC’d.

What I can’t figure out is how to interrogate each PO in a script on the”Bill Purchase Order” form when several vendor POs are check marked.  Any ideas?  Also is the correct script deployment application Vendor Bill?