You can check your client balances by heading to the 'Finances' tab.
In this area, you'll see a full list of your clients and whether they have an 'Amount Due' owing or are 'In Credit'.
If you want to check the balance breakdown (i.e charges for each session/class and payment transactions), click the magnifying glass icon on the right side of your client's name:
In the transaction view, you can find all the charges & payments which make up the balance:
Related articles:
How to record a payment transaction
How to send payment receipts to clients
How to create and send client invoices
How to record and track expenses