You should perform a vacuum box test for leaks and magnetic particle (ferromagnetic materials) or liquid dye penetrant for cracks and other welding discontinuities.
API 650 and 653 give guidelines for what types of inspection are required.
These are the most practical and cost-effective, however; there are people out there who will try to sell you some high-tech super-duper thinga-ma-jig that you really don't need. It is best to just stick with the basics....
The APIGUY....