There are many types of NDT..Red dye is the quickest and cheapest method of inspecting for cracks. If your cracks are hairline and very "tight", the red dye might not allow full penetration. that case, its best to go with a secondary inspection such as ECI or UT for concurrance.

- liquid penetrant (PT) red dye or zyglo (FPI)
- magnetic particle (MT)
- eddy current (ET)
- ultrasonic (UT)
- radiography (RT)

Heres the basic list of different inspection techniques:
Magnetic Particle Inspection
A nondestructive test method which provides for the detection of linear, surface and near surface, discontinuities in ferromagnetic test materials.
Liquid Penetrant Inspection
A nondestructive test method which provides for the detection of surface discontinuities in ferrous and nonferrous test materials.
Radiography Inspection
Radiation used is higher energy version of the electromagnetic waves that we see as visible light. Radiation emitted fron an x-ray generator will penetrage the sample which will identify cracks
Eddy current Testing
ECI is well sutied for detecting surface cracks and measure coating thickness.
Ultrasonic inspection
high freq sound waves are introduced into material and are reflected back from surfaces or flaws