Vesselfab is correct. If the material is carbon steel, RT before PWHT can be performed or after PWHT. I would have a wet fluorescent MT performed of the weld region if RT is taken credit before PWHT. Cheap insurance.
In my opinion a good practice is to have RT also after PWHT becasue it could make some defect bigger. It is more expensive and so you have to decide if it is necessary or not.
As vesselfab indicated the code looks for the end product by checking it with proper NDTs. However, the respectable manufacturer companies ask the subcontractors to do additional tests before the PWHT to make sure the welds are going to pass the full radiographic and magnetic particle tests.
Additionally, the plate material should be adequate by ensuring the suitability by asking additional test in the purchase order. If you look at the ASME material standards, there are some nonmondatory requirements (S1, S2, S3, etc...) that purchaser can ask on the plate. All of them are to make sure the end product is adequate and in accordance with the design.
Otherwise, it will cause a lot of problem if yo find out some part or entire welds are not adequate after the PWHT. Each repair will/may require another PWHT and the same NDT. So, it will become an endless exercise and very costly.
