Undo the bolts that hold the coils in place, lift up each coil individually with the engine running.

When you lift the faulty coil the engine note will not change. When you lift a good coil the engine note will alter and you can hear the spark tracking to earth.

I use insulated pliers for this. At first I was a bit concerned of potential damage to the ECU but in over 10 years of doing this never had a problem.

Nick