1. Check voltage to input of coil with switch on. Should be near battery voltage. If not there trace back to battery, if ok go to 2
2. Check voltage at output of coil to distributor. Should be battery voltage with points open, zero voltage with points closed. If constant battery voltage look for bad connection in distributor/points circuit. If constant zero voltage look for short in points, condenser, or distributor side feed through or an open between + and - posts of the coil.
this assumes you have a 12 volt - no resistor required coil. If it has coil with ballast resistor check the resistor.