Explain electric ignition to me...

for testing these coils....

there should be 3 total wires on it. Basically these are identical (in wiring) to all ignition related coils. Heh, even the Model T coils were 3 wire.

Anyhoo, when you go to test them, it may seem hard to figure out the connections. The 3 wires are; Positive (+) low voltage, AC High voltage out and a common ground (both coils share ground).

You can check resistance several ways, the easiest way to check HV integrity is as you mentioned; from the HV insulated lead (doesn't have to be on the plug, could be socket) to ground.

If you have the whole coil off then you can check the individual leads. Because the 2 coils share a ground, you can actually get resistance to show on all leads. I believe the CDI has 2 wires to the coil (correct me if not). One of those is positive, the other is the common ground.
The only trick when it's completely unknown, is to find the ground wire. Once you do that, the other two are simple to identify.

I don't have one of these to check their proper resistance, but I could throw some assumptions out for testing examples.

the low voltage coil- should have a low resistance probably a few milliohms.
the high voltage coil- should have more resistance probably a few ohms to maybe 15 (doubt that high).

** if all wires are unknown and you check (accidentally) between Low voltage positive and AC HV output; you should show a resistance of BOTH coils added together.

Finally, if you get "proper" resistance on the coils; then there is no short (break) in the coils. This means the coil is functional. If you know it is being driven correctly (e.g.- CDI working and magnets good); then a weak spark most likely means there has been an insulation breakdown between wires.

If you have multiple fail from insulation breakdown, then that's probably in the coil design rather than a "bad one".

Hope this helps