Mikey to find out which coil(s) is down manually, remove the engine cover, remove the 4 bolts holding the coils in, but leave the coils there. Start it up, and in turn, lift each coil, if you can remove it all the way without it making any difference to the engine - thats the duff one.
You should also be able to hearing it 'sparking' when its just lifted from the top of the plug. Theres a few other ways of doing it, e.g injector plug/clip computer misfire rate. Inspect them for cracks aswell, arching against the head.