You can check resistances in the coil windings: First, put your meter leads together and read just the lead resistance; do that several times and average those lead resistance readings. Then measure resistance though the coil primary; check + to - terminals with at least one of them having the wires disconnected. Then subtract the lead resistance from before; primary resistance should be around 0.4 ohms. (It is not easy to measure this low resistance accurately with normal meters...)
Secondary resistance from coil - to the spark terminal should be up around 4000 ohms.
Note that resistances can be good and the coil can still be breaking down under high voltage. There often is no visible clue to this.
Check the lead resistance through the spark wire from the coil; it ought to be no higher than a few thousand ohms.
Reinstall the spark lead on the coil and put the other end 3/8" from metal; it ought to jump that size of gap easily with a snappy blue spark, and do it steadily while cranking.
Recheck under cap for any carbon tracks on the inside surface.
