If you want to check your debitor bills inside your app you can do so by changing into the admin view under Admin -> Manage Bills.
Debitor bills can be distinguished by the 'customer' symbol next to the bill number. You can see the difference between a regular bill and a debitor bill in the following screenshot:
Important: It is not possible to change the payment-method of a debitor bill via 'Change Payment-Method'. The only way to do this is via 'Save Table' and repayment of the table with a different payment-method.