To see the debt of a customer we can go to Customers -> Customers .

There we will see the list of our customers, then we can press on  to access the client's file which we want to consult the debt

Once in the client's file, go to the left menu and press on Billing and collection, then select the Debts tab

There you'll see the list of all the debt of this customer.

Also, as soon as you enter on the client's file, If he/she has any debt, the system will warn you about it with a pop up window showing the amount, and also you'll see the traffic light on the right in yellow / red and the amount of the debt there as well. If you click on the traffic light you can see what kind of debt the client has in a windows like the following one

If you want to see the debt of all your customers, then simply go to  Inf. and Statistics / Customers / Customers with debts

Once there click on Process to get the list of updated debts