Lay a piece of sheet metal on a table to simulate the car frame
Battery next to the sheet metal
coil next to the battery
Coil Hi tension to a spark plug
Lay the wide hex of the spark plug on the sheet metal

Batt +12 connects to the coil +
Batt - connects to the sheet metal


The coil - connects to a screwdriver shank
Touch the screwdriver tip to the sheet metal. When it grounds 
the coil, the primary winding should shoot a hi voltage charge 
to the spark plug

This doesn't mean the coil is good.  Many times a coil can start and 
run an engine fairly well, until a load is encountered.
A weak coil will fail to deliver an adequate amount of voltage to fire a plug 
under load.
 Coils are usually laying around the shop because the owner wasn't sure it was
 the problem, as they are usually swapped out with the plugs, points or a fuel pump, 
in the quest to find why the engine was stumbling, going up a hill.

Just toss them all and buy a new one, if you need it...