Dye Penetrant Inspection (DPI) is a widely applied, low cost method of detecting surface breaking defects in any non-porous material.
In a nutshell, the process involves spraying the material with a penetrant, allowing time for this to soak into any cracks that may be present, then cleaning the material and allowing it time to dry, this can be done using either a cleaner, or if the penetrant is water washable, just water. The next stage is to spray the material with a developer and wait. The developers job is to draw out the penetrant so that the crack size and shape is detectable in the material.
Items related to this area include (among others): Red Dye Penetrant, Cleaner & Developer