According to mm :
Have you tried using a spark plug as a tester? Unscrew it from the
negine, connect it back to the wire, and have someone hold the metal
threaded part against the block, or use a wire with aligator clips on
each end.

This is a reasonably reliable (and easy) test. However, it still may
not be enough spark.

The proper way is to see if the spark will jump a full 1/4". Our
small engines instructor has a tester made in the following way:

Take a spark plug and saw off the threaded portion, leaving central
insulator and spark gap center pin intact. He then fastened a small bracket
with a screw in it, so he could adjust the gap between the screw and the
spark gap center pin. A grounding strap with alligator clip is then
fastened to the body. Attach the clip to the motor frame, the spark
wire to the top of the modified plug, and see if you can pull a spark
across the 1/4".
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.