What are the signs of a broken down coil? can they be tested?

My car has a mega flat spot between 2 and 4 k ??? when it was rollin roaded yesterday, engine light came on and found yl 4 misfire, fitted new plugs today and misfire not as bad tho has this flat spot, swapped the number for coil to number 3 now...

 

??

 

coilpacks can be tricky little things, my experience is they can be tested if in singles i.e classic shape but i couldnt tell whay reading youre looking for but im sure theres a write up on it here somewhere. also worth checking your earth connections for corrosion etc, although that would have indicated a miss on cyl 4 and 2 but not if single, cheapest option is to hope it throws a light again and detects a miss on 3 now then just renew the coil. quite often fail in pairs also..hope this helps..

 

will check, swaped over just waitin for a mate pop round plug it in... remember on my saxo did the same kinda thing and was coil, also vag cars suffer from coils lots too,...

 

anyone have any coils spare?????