There seems to be no way to determine whether a purchase document (i.e. a P.O or a Purchase Invoice) has been generated from a sales order. If one has been generated, there seems to be no way to determine which one it happens to be. In experimenting with this functionality, I can also create any number of duplicate P.O.'s or Purchase Invoices from the same sales order.
After I've created a P.O. (and especially a Purchase Invoice) directly from within the Sales Order screen, I should at least get a warning if I try to create a duplicate. I should be able to navigate from the sales order to the applicable purchase document. I should also be able to navigate from the purchase document to the related sales order, as well.
This kind of functionality is essential for managing purchases when these are directly related to sales. This sort of functionality is basically available for drop shipments, and should be available for normal shipments as well.