Cracks would most likely occur on the exterior of the mast where the tensile stresses are the highest. You can use a visible (contrast or nonfluorescent both mean the same thing) liquid penetrant applied to the exterior surfaces. The manufacturer usually includes sufficient instructions to perform the test for your own information and satifaction.

The alternative is to have a "laboratory" perform the test, in which case a fluorescent dye penetrant test is recommended. It will cost you, but the test will be more sensitive to very small fatigue cracks.

Best regards - Al