How can I identify which Supplier timesheets have not yet been paid?

The Unprocessed Supplier Timesheets report within the Supplier Payment menu is the best way to identify which timesheets have been validated for payment but not yet paid:

Set the filters at the top for either Purchase Invoices or Self Bill type, then state if you wish to see only timesheets that have already produced a sales invoice (billing).  This report (as with all reports) can be exported to Excel and filtered as required.