Oscilloscope for testing
 

Oak had asked for some information regarding o-scope use to test ignition systems. While I am no electronics genius, I will share what little I know.
An oscilloscope by definition - " a device used to observe the change of an electrical signal over time, such that voltage and time describe a shape which is continuously graphed against a calibrated scale. The observed waveform can be analyzed for such properties as amplitude, frequency, rise time, time interval, distortion and others by a display on the screen of a cathode ray tube " (Wikipedia)
What does all that mean? By using a o-scope, you see on a screen, what the electrical signal, such as the surge of current to fire a spark plug, looks like. Using the display and the correct settings you can measure the actual current as a waveform. Think of it as a multi-meter that shows you the readings with pictures in addition to a numbers display.
If I had I used a oscilloscope to test the coil output on the skipping M18, I would have been able to see the difference between the two outputs of the coil while the engine was running. When the coil misfired, it would have shown up as a shortened waveform on the bad wire as compared to the normal output on the good wire. Both waveforms would be displayed side by side.
There are many different levels of scopes, from cheapies to full blown, super accurate, every function models!

Here's a link to a very good learning site about Oscilloscopes -----
https://learn.sparkfun.com/tutorials...n-oscilloscope

I welcome others to add better descriptions.