To find out the rest of the payment, you need to open an order and see the Balance.
The positive balance is displayed in black. This means that the customer has overpaid this amount.
The negative balance is displayed in red.
This means that the client needs to pay this amount.
The zero balance is also displayed in black.
It shows that the customer has fully paid for the services.
You can also see the payment information on the Payments tab.
See the balance directly from the order table.
How to set up the table, see here