Using the Debtors List Report you can find patients in credit.
Go to Admninistration->Reports
Choose Debtors List
On the select patient box, create a new query (Creating and Editing Queries ) for "patient balances" and make it from -£9999 to -£0.01 (you can also use this an create a contact list instead if preferred)
Under minimum amount leave it blank
This will show all patients in credit (negative amount).