ADN;

Checking for high voltage leakage external to ignition components is very effective at night as Chris suggests.

You can unconnect, one at a time, plug wires and plug them into a spare plug which you then lay on the valve cover. This allows checking for spark with the least intrusion on the system.

Also, if engine starts on three and smooths out after warming, start it, and shut off after 30 secs. or so and (carefully)feel for heat at exhaust...this will locate the nonfiring cylinder...try replacing plug first and try starting again, if no improvement, check plug wire and cap, especially at problem cylinder connections.

Good Hunting