Improvising a spark plug indicator light ?

If you have an oscilloscope, you can wrap a few turns of wire around the ignition cable (as a quick inductive pickup), then hook that up to the scope.

I agree with tcmtech the neon bulb is probably the best to go with. This is on a small Honda generator running with a load on it, in daylight its fairly bright to me, & these bulbs can be salvage any where for free.

debe, what is the brightness of the neon bulb compared to the xenon bulb without the added "primary" or low voltage circuit? By primary/low voltage I mean like when used in a photo flash or powered timing light.

If its an engine made after the late 90's its probably a wasted spark, a 4 cyl engine h=has 2 coils, one end of the ht goes to one plug, and the other end goes to another, one plug fires the mixture and the other plug spark is 'wasted'.
This being the case you only really need 2 indicators.
A shorted or fouled plug will still show good on something like this, a capacitive pickup and an automotive or usb 'scope tells you a lot about the ignition system and whether the engine is running well or not with a little practice.
Nice gizmo debe, I have a commercial one similar, also have one with an adjustable spark gap to 'work' the coil.

shortbus after trying both, the Neon bulb was mutch brighter when being used like this.