Once you login to your account simply click in the top left hand corner "My Account" then "Billing".
At the top of this screen you will see your account balance. You can see an example of where this is below:
Important: Your account balance only includes commission that has had an automated self billing invoice generated for it.
What does this mean?
This means any commission you have earned in the current month and potentially the month before may not yet be included in your "account balance". Your account balance is effectively commission which is ready to be paid out, but has yet to be paid out.
Any commission from the current month (and the previous month if a self billing invoice for the previous month has yet to be generated) will not be shown in this balance. Self billing invoices are usually generated between the 10th-15th of the following month, once a self billing invoice is generated any commission covered under this invoice will then move into your account balance.