I'm no expert but I'll give this a shot. First, get an ohmmeter and remove the coil. Ok, in the coil, there are 2 windings: a primary and secondary.
The primary winding is pretty much always good but check it any way as follows: set meter to 1000 - attach one lead to the green wire - the other lead to the ground wire - the meter should read off the scale.
The secondary winding is most often where a bad coil shows itself. Keep the meter at 1000 - attach one to the spark output of the coil (don't ground out on the sidewall - you want the centre pin) and attach the other lead to the ground wire of the coil - if the meter reads less than 7 ... it's junk. New ones tend to read just over 10. If you get a reading you're happy with .... attach the spark plug wire and put the lead to the spark connector in the rubber boot - if the reading is pretty much the same as you got from the coil output pin itself you know the spark plug lead and connector is good. If there's a big drop, well replace the wire and connector.
I guess I should have said that prior to all this - give the coil a good look when you remove it - if there are any cracks whatsoever ... time to replace it.
