I have kept some known good secondhand coils when I have replaced sets. First thing I do is stick a scanner on and see if there are any misfire codes logged (bearing in mind that No 1 is the gearbox end). Then I fit a known good coil to confirm it, then I get a new one in.
On several occasions I have had No1 or No2 plug wells full of rusty water but have never found out how it gets there, I think it may be rain water run off from the scuttle but I am not sure.
On one occasion very recently one coil (No2) was badly water damaged and still working but No3 was the faulty one. I wonder if water temporarily shorting No2 out caused No3 to be damaged, the pair were replaced.
Shaun