Customer credit limits can be found in
Sales
Customer Maintenance
Clicking Sales Orders tab.
Customer credit limits are for information only and cannot be used to prevent the creation of Sales Orders due to the number of conditions and human choice which exist for Sales Orders.
One suggestion which could help if you would like users to be more aware of the credit limits would be to set up a Sales Order Message which would then display a pop-up message each times a Sales Order is created for that particular customer.