My daughter's car wasn't starting and this little $2 tool from Harbor Freight (see attached photo) helped us diagnose what was wrong (the coil output was not quite enough). It plugs in-line into a spark plug path and lets you see if a plug is firing. It's just a small incandescent bulb in a plastic package. I also used it this afternoon in getting the riding lawnmower running. It wouldn't start; the tool verified that it was getting a decent spark. Some more cranking and starting fluid finally got it running.

Oh, and it's probably more ethical than plugging your mother-in-law into the spark plug wire and grounding her. Not as fun though.

 

I think a small neon lamp, or incandescent bulb with a couple wires, would do the same thing, right?
or stick a wire in the spark line and see if it will arc to ground?

[EDIT]
that wasn't to criticize, just asking

 

My daughter's car wasn't starting and this little $2 tool from Harbor Freight (see attached photo) helped us diagnose what was wrong (the coil output was not quite enough). It plugs in-line into a spark plug path and lets you see if a plug is firing. It's just a small incandescent bulb in a plastic package. I also used it this afternoon in getting the riding lawnmower running. It wouldn't start; the tool verified that it was getting a decent spark. Some more cranking and starting fluid finally got it running.

Oh, and it's probably more ethical than plugging your mother-in-law into the spark plug wire and grounding her. Not as fun though.

I'll pull the plug, reconnect and lie it on the head and crank, looking for spark. But it's not just the spark I'm looking for. The plug indicates current combustion condition, it's removal assists in cranking at speed, and it lends to a compression test before and after a shot of oil.

 

When working as a mechanic i had a new sparkplug in my tool box with a 2mm gap. This was pluged in to the lead & sat on the cyl head, when cranking it should produce a fat Blue spark. A Yellow spark is weak & unlikely to fire the mixture under pressure. This works for me.