Typically, in the auto industry if there is a design change or welding equipment (incl. wires & gas) change there is a requirement for pre-production testing of the product. From what I understand they will specify the type and number of tests based on the application of the part as well as the number of parts to be produced.
Also, they generally require that you take random samples from your production runs and test them as well.
I would ask your customer what type and quantity of tests that they will want.
-Sean