mmmm..
I don't know. To be honest, I never tested a ICU-S ignition. Still use the good ol' Vlach myself.
I assume, with a bit of background knowledge, that if the ignition produces a spark over 6mm (out of the engine), there is no sparking inside the unit.
I have a test bench I can run at 7000 rpm, that has this free air gap. I use it to test all kinds of ignitions. You can simulate it by taking an ols spark plug, take off the ground electrode (or bend it open) and see if there is a strong spark. If there is, all seems well.