What checks be made before user finally closes a PO?
Does system make any checks and gives any warnings when user tries to finally close a PO?

Systems Does not check Before User Finally close the PO.

check that all the Shipments which are received are Billed and there are no
Receipts Pending for Invoicing. If the Receipts are Pending for Invoice then
the Invoices will be rejected while importing them into the Payable