In addition to the above suggestions...
Run the engine in the dark. Look for any sparks at the wire boots. Look for sparks where anything touches the wires.
Turn on an AM radio. Bring it close to the engine. THe static will be severe. Listening to it can tell you something.
If you have an oscilloscope, you can test whether all plugs are firing. Do not make any direct electrical connections (unless your scope handles hi voltage!). Simply use a clothespin to hold your scope lead against the center wire at the distributor (rather against the wire's insulated part).
Adjust sweep speed so the screen shows the same number of pulses as you have plugs. An misfires will be obvious. This will alert you to a bad plug, or bad wire.
Disclaimer: I am not responsible for what happens to your oscilloscope. I have an old tube type, AC powered, and I got no problems from doing this.