The most common non distructive tests (NDT) for pipe fittings:

Positive material identification (PMI): identification of the chemical composition of the metal used to manufacture the fitting. Uses PMI sensors, including X-ray fluorescence or optical emission spectrometry. This test is frequently executed for high value materials, from duplex steel onwards.
Penetrating liquids: this is a low-cost test to detect cracks or abnormal porosity on the surface of the fittings, and is executed by applying a special liquid to its surface.
Ferrite content: this test aims to measure the ferrite content for duplex, super duplex and stainless steel materials to make sure the metal is able to achieve the yield strength, fracture toughness and corrosion resistance it is meant to reach. Ferrite content is measured in weld seams by magnetic induction.
Hydrostatic test / hydro testing (pressure testing): the fitting gets filled with a test liquid (example water) and a pressure is applied to check if any pressure loss occurs (which would indicate manufacturing defects)
Magnetic particles: by means of a magnetic field, possible surface discontinuities or weakness can be identified
Macrography: 2 / 3 D pictures of items visible to naked eye.
Micrography: pictures at the microscope to see the microgranular structure of the metal and other details not visible at the naked eye
Pneumatic pressure: the fitting gets filled with pressurized air; measurements are taken to see if there is pressure loss due to any defects.
Radiography: X-rays / gamma rays to detect imperfections leading to possible quality defects
Out of the above tests, the pressure testing is one of the most commonly specified, especially for mission critical fittings in the pipeline.