There are some hints of progress that sometimes occur, in the process of getting it started:
If, when you are turning it over, it starts to pick up speed, even if you don't actually hear it firing, you are very close to success.
Sometimes it will fire and it sounds like a muffled thump. This is also a sign that good things are happening.
Other Hints:
Sometimes the starter can draw quite a lot of current and this draws the voltage down quite a bit. The end result is the spark is affected. You can power the coil from a different source, by disconnecting the ignition feed from the dash and connecting directly to something like a battery charger.This will give you the hottest possible spark all the time, regardless of the starter draw.
The spark you see in free air is not the same as the one in the motor. Inside the motor, you have a very dense mixture of air and fuel at a high pressure, and it is much more difficult for the spark to jump. Old plugs can definitely fail because of this.
