For internal Flaw/Imperfection of a materiel, a good way to examine is by using Ultrasonic Flaw Detection. It transmits a high frequency of sound waves into a material to detect imperfection. The sound is introduced into the test object and reflections are return to a receiver from internal imperfection.
Ultrasonic Flaw Detection is able to detect internal imperfection of object compared to Liquid Penetration Testing & Magnetic Particle Inspection.
The test surface must be clean and smooth, any paint are to be removed. Imperfection/Flaw cannot be visibly seen, it is determine by the echo that returned to the receiver.