Detecting sparks at the output of a high voltage flyback transformer
Do you have 300MHz BW on your scope?  That's what you will need I suspect.  See my other answer on what to expect. Parasitic Capacitor current of an inductor  Put one or more turns around the secondary into high quality coax ( not RG58) or SAT. coax and terminate with 50 or 75 Ohms to match the coax.  This is a 1:1 CT. 

The energy stored into the falling dielectric starts as a rising voltage relative to the kV/mm in the plasma , then it breaks down and explodes inspite of the low capacitance but large V^2.  The plasma waves are semi-conductive ( negative resistance ) rather than insulative air dielectric.

Then use a peak detector that has 300MHz BW (CML comparator) to amplify and peak detect the pulse with a low pF high speed diode to small cap to stretch it but not current limited then out to trigger a lower (20MHz) BW 1-shot to 1 second.  

There may be a PD pulse before a full breakdown arc and if you are fast enough you can divert the energy, somehow.

For commercial and researcher solutions, use google scholar on "PD detector" which tend to cover a wide range in costs. You just want to detect the secondary stored energy pulse which does not show up on primary.

My signal came from an invisible contaminant in a jar of oil at about 20kV with a 1mm gap.