If I would need only to detect spark, I do not disturb you! I have to sample this signal and analyse his shape. So, signal must be feeded to ADC. I am shure, that simple voltage devider is too dangerous, and it will affect the process - the shape will be distorted - the spark is between air gap with resistance millions of Oms, and my devider will be with less resistance, so there will be no spark any more :(

What if I'll use current transformer on the HV cabel? Of course, current is very small, but it can be detected anyway?