Confidence in reading secondary patterns comes from experience. I do not have much with 104 motors. I have used inductive pick-ups somewhat like the old GM HEI distributor adaptors most scopes used to have. The patterns have been poor and only one at a time, so I haven't liked using them for the normal things I would do with secondary. So far the best results have been to look at the primary and/or to current ramp the primary. I just bought one of the new Vetronix MTS5100 scopes and will try and see what I can get hooking to the plug wires. I presume that this is like waste spark systems and both plugs should appear on each wire. The question will be what to use as a sync pulse.
Bosch Master, ASE Master, L1
33 years MB technician