We would like to enable the quick purchase order registration only for small POs.

Eg. up to a value of 100 Euros. 

Is there a way to find out if a PO was created through the Quick purchase order registration screen? And  

for example create a small event on this which blocks the purchases > 100 Euro?

I cannot find any field which indicates the PO was created through the quick PO registration screen. 

So only option looks like to add a custom field on the PO header which is only filled in when you create the PO through the Quick purchase order screen?

Or is there an event on which we can listen to verify the total value of a PO created through Quick PO registration?

Thanks!