Common mode inductors with high pulsed current
If you are winding your own, you can use bifilar wire which when not twisted on a toroid is pretty nearly perfect.  Otherwise you can expect to see the effect of a fraction of a turn; if you use one turn at your maximum current to specify your toroid you will be safe.  Wind in bifilar fashion in any case for the best results.  If you wind one winding over the other the results will not be as good.

You can measure the results directly with a third (temporary) coil of fine wire, short out the output and drive the input with some AC.