How to tell a Scratch from a Crack in a Cylinder?

Looks like a scratch to me.
Magnaflux would most likely show an "indication" because the magnetic lines of flux would obviously be disrupted there so I'm not sure that would tell you anything...just scare you. Dye penetrant would indicate if there is any depth to it beyond what you could see due to the fact it would get sucked out of the depths of the crack if there was one, when you apply the developer.