You will be able to check your clients' outstanding balance by these steps:
On this area, the full amount of your clients' outstanding balance will be displayed.
If you wanna check the balance breakdown (charges for each session/class and amount of payment made for each transaction), click the 'magnifying glass' on the right side of your clients' name. You'll then be routed to 'Transaction' submenu.
In case that you need to change any information from the transaction shown, just click the 'Edit' button located at the right side of it.