To see how much credit a Client has, go to People > Clients > select the Client > Accounting. Any Credit Request payments they have made will be reflected in their Invoice Balance. Read more about Client Balances here.
An example of a Client’s account balance with the Invoice Balance section highlighted.
Note: a paid Credit Request raises both the Invoice Balance and the Available Balance. The Available Balance then falls as Lessons are completed, while the Invoice Balance only falls once an Invoice is raised — so the two figures will often differ. See here for the difference.
To see every Client's credit in one list instead of checking them one at a time, go to Accounting > Client Balances — see here.