How can I tell if a purchase order is a drop shipment or special order after it has been fullfilled?

The Ship to address: Drop Ship is customer's address, Special Order is your company address.

You may also be able to look at the Custom Form on the Purchase Order as well. Name might contain "Drop Ship" or something to that effect. Special Order may use the same Drop Ship form as well.

It can also be done with a search: Applied to Link Type