A quick way to see if your invoices raised are paid or unpaid is to run a report of Invoices raised within a time range and see which ones are marked as payment 'Received'.
Go to Reports, click on Invoices raised. Select on the date to run it from, then click on Generate Report.
The report will show the list of invoices raised within the time range you set. At a glance, you'll be able to see if a payment was received or not for each invoice.
The payment will either be 'Marked as received' (where user manually clicks on the 'Received' button in their Invoices dashboard) or 'Received via Okappy' (customers pay by clicking the green 'PAY INOVICE' button via Sage Pay).