You use an inline spark plug tester.
I use the neon tube kild because the red flash is easier to see.
They are fairly wel proportional.
Bright red flash = good spark
Faily pink flick = poor spark
and of course, no spark = some ones stolen your bike
You can use a biro tube tester ( or buy one from a mower shop ).
This is also an inline do dad which has a sliding contact inside a clear tube.
A good spark will jump 1/2 gap between the contacts in the tube.
From the shops that sell foxtails for you car radio areils and lights that illuminate the road under your car you can usually get flashing spark plug caps.
Again a type of inline plug tester converted into a must have for car drivers who walk around with both hands in their pockets and a fuuny smile.