Hmmmm.
The easiest option is to test with a known good one - that works well in a shop, but not in your garage at 10 PM when you want to get the saw going. The other option is just to try it - pull the saw over and see if you get a big fat spark. As long as the points are moving an not gummed up, you should get a spark.
Pretty much all coils are simply two sets of windings wrapped round a laminated iron core. The low voltage side has a small number of windings, the HT side has lots of windings. Roughly, the low voltage side is "charged up" by applying 12 volts across it, then the power is cut (by the points), and the decay of the voltage induces a massive voltage in the HT side - hence a spark.
The electronics is all in the low voltage side - HT and electronics do not play nicely.
Step 1: Visual inspection. If the thing looks like it has been at the bottom of the sea and the laminations are all splitting apart, it will deliver a weak spark. Replace.
Step 2: get an ohm-meter and clip one terminal to the iron core, and the other to the HT lead (where the spark plug goes). You're now testing the resistance of the secondary coil. Open circuit = a broken coil. Zero resistance = a short to ground. Both of these mean it is a dud. You're looking for a resistance of something like 100 - 10,000 ohms. It varies by coil, and will be published on the web somewhere.
Step 3: there will be another wire leading out from the coil - this is for the kill switch. Move the ohm-meter connector from the HT side to this wire. Zero resistance = a short to ground, and a dud. Anything else - who knows, this is where the electronics kick in.
If you want to really have a look at what is going on, you can attach a scope to the HT lead of a running engine, and watch the build/decay of the secondary voltage. More here:
http://www.picoauto.com/tutorials/primary-vs-secondary.html
I tried it on an 090 but the points were so erratic that the scope could not lock onto the waveform. I'll try it with a more modern ignition some day.
