According to Gideon :

6) Test the coil output by inserting a nail into the spark wire
boot and then hold the other end of the nail near the block
while cranking the engine.
7) Test the spark plug outside of the engine by placing it
against the block and testing for spark. A plug will fire
more easily in the open than it will in the combustion
chamber. This helps determine if you are getting any
coil output.
8) Try temporarily reducing the spark gap to about 1/2 of
the factory spec. This makes it much easier to get a
spark from a less-than-perfect system. This is a good
diagnostic. Also, an engine can be run this way, although
it is a somewhat less than optimal situation. The smaller
spark impacts engine timing, but not in any destructive
manner. (to the best of my knowledge).

As a FYI, our "small engines instructor" says that the simplest
way to test for good ignition is to "construct" a 1/4" spark
gap, and you should get a reasonably fat/blue spark off it. Anything
less than that indicates you're probably not getting a healthy
enough spark, EVEN IF, you can see it on the much narrower spark
gap on a plug.

He "constructed" this tester by taking an ordinary plug, and sawing
off the threaded portion, leaving the center pin open on the end
of the ceramic insulator. Then, installed a clip on a wire
to ground it, and a screw to adjust the gap from the center pin
to the center pin. Install the modified plug into the spark
wire, the clip to a ground, and pull...
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.