To view balances for un-used carry-forward credits, you will need to enable Credit Invoices for your site first. Please read the article HERE to read more about this feature before enabling, if you haven't done so already.
Once you have enabled Credit Invoices in your site, you will be given access to new fields in several exports, as described in the Credit Payments article here.
Specifically, you will want to use the invoice_balance column to view unused credit balances within the Invoices - Summary export.
The invoice_balance column shows the current balance of the charge or credit invoice.
Please note that this field will always show the current value of the unused credit balance. It will not track historical values, which means you will not be able to see what the balance was on August 1st, for example.