I have a twist bbl which I "Know" has about a ¼" long crack following the twist. Knowing where it is I can only find it with the aid of a magnifing glass. Anyone who didn't know it was there would never find it from a visual inspection. I have no idea if magna-flux would work on it or not. I would presume if properly set up by a knowledable operator it would. Possibly dye penertrant & a black light would also find it.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra