A plug that is fouled will allow the spark to travel down the sides rather than jump the gap & ignite the fuel.
The lazy way to check for fouled plugs used to be to pull the cap off and hold it 1/2" to 1/4" off then end of the plug.
If the engine changed speeds, that plug is bad.
The old engine analysers you used tom see in garages with plugs & wires every where did exactly this , electronically.
Now days the Engine management system does the same thing.
Back in the 60's to 90's you might remember all of those Spark Intensifiers & Power Increasers and other "miracle power increasers "
All of them , apart from being snake oil which caused premature failure of coils & condensers, were nothing more than a gap in the spark plug wire.
Back in the 10's through to the 50's the bodge was to get a biro tube , cut the HT wire(s) and shove each end into the tube with a gap between them.
Then there was a hose with a fireman's brass button on each end of the wire, all sorts of cures & improvers.
