Since we get asked a lot, here's a very clear video from the folks at Rocky Mountain ATV MC on how to test your ignition coils. 
One additional thing to keep in mind when testing is that if you are testing with the resistor caps on the wires, make sure you have a good connection between the wires and the caps, as well as a good connection between the wires and the coil itself. If in doubt, unscrew the caps and test directly to the wires, or remove the wires and test directly to the coils. This will also identify problems with the wires or resistors.
All used ignition coils that we sell at HoopRider Parts have been tested and are good.