Golden weld joint is that joint in which the new piping is connected to the existing piping. Hence this joint cannot be hydrostatically tested. So it has to be ensured that it should be defect free as per the standard (eg: ASME B31.3 Section 345.2.3).
Welder assigned for this joint is a very important criteria. He have to be highly skilled as the NDT requirements for this joint is normally higher than that of other joints.
My experience, the golden joints are subjected to RT, UT, MPI & hardness test. It depends on the standard being followed and the client requirements.