12 volt or 6 volt coil how to tell difference

[johnbuckley]

Ok you lot, Slow witted limey here... most of us have kicking around our workshops a variety of coils like this....
a) coils for 12 volt non ballasted ignition circuits, b) coils for 6 volt ignition circuits, c) coils for 12 volt ignition circuits with a ballast resistor. d) so-called sports coils for 12 volt circuits.
Can we get a definitive answer between us as to how to identify each of them?