I partially concurr with ANJAFO. Magnetic particle inspection [MT, MPI] would be the fastest way to verify surface-breaking defects in ferritic [magnetic] material.
However, Visible dye liquid penetrant [PT, LPI, is the best method for evaluation of visible [possible] cracks. If the defect is verified, you can grind into the defect and reapply developer to visualize the defect. You keep grinding off a little and redeveloping to slowly 'gently' explore the defect. This exploration allows you to find the depth of the crack, determine both ends of the crack, and get a 'feeling' if the defect was indeed a crack, or a forging/rolling lap, or was a bad weld seam.