get an old spark plug, remove one of the leads and plug in the old plug. place the old plug on the head so that it will spark (dont hold it coz it might hurt) and get someone to turn the engine over. Repeat for the other leads.
If all 4 spark there is another fault.
If only one doenst spark then the coil is faulty.
If two on seperate coils dont spark then possibly both coils are faulty.
If two on the same coil dont spark then its possibly the ECU. Swap the wire from the ecu to the other coil i.e. the one that does spark put on the one that doesnt (dont put the other lead on the other coil) turn the engine over again and if the same coil doesnt spark then its the coil if it does then its the ecu. However some times the coil fails and damages the ECU.
I had a missfire on my punto and it turned out to be one of the valves sticking open.