If you just want to detect when the spark takes place an inductive pickup is a very reliable and trouble free method, professional time strobes used to use this method.
To measure the ignition voltage you could do it with a string of resistors as a divider into a op amp circuit, but it would need to be high impedance to avoid pulling down the ignition voltage, it would also need to be low capacitance to avoid changing the coils resonant freq which would give you misleading results as the voltage might well change with freq.
Maybe you could do something that detects the electric field, shouldnt be too hard with ht, maybe a pcb track carring the ht voltage with a parallel track going to a fet or something on those lines.