Lits of Items ordered but not delivered

What is the way to see items that have been ordered but not yet delivered?

We created Purchase Invoices with many line items. The supplier then often just delivers some line items at a time and bills us for those. At that point, we create a Payment Entry and a Purchase Receipt for the items we received. So the PI’s are partially paid. But so far I haven’t found a way to see which items from the open PI’s still need to be received. We are in the electronics industry and the lead times for components can be very long, so sometimes a supplier will deliver a part a year or more after we created the PI.