If you assume the strain in the extreme compression fibre is the same as the strain in the extreme tension fibre and that it is zero at the neutral axis, then you can relate the radius of curvature to the strain in the steel and compare that with the minimum strain that your particular steel is required to comply with eg 0.15. (Of course, the above asumption may not be correct, eg it may have more strength in compression than tension, and the bending process may restrain the compression fibres).