Use a test light, and attach the pigtail wire to a known good ground such as the engine block, etc. Touch the probe to the distributor side of the coil primary. (Usually the - terminal). With the points open, the light should burn. As soon as the points close, the light goes out. If the light burns all the time, you've got dirty points, wrong point gap, a broken primary wire, or some other reason the voltage isn't getting to the points. If it's off all the time, points open or closed, you've got a short to ground somewhere between the coil and the points. The light should flicker on and off as the engine is cranked if you probe the distributor side of the coil with the test light.
Jerry
