Any tricks for finding the path taken by small ESD sparks across a PCB?
Do you have a blank board, no components?  That's how we usually do it.

Also, what is the source of the kilovolts causing the sparking?  Is this your calibrated pulser during an EMC test?  Or HV crap coming into the board from long comm lines?  Or a 50KV environment such as industrial conveyor belts and rollers?

With a DC HV supply, apply slowly-increasing kilovolts (with current limiter) to the wires soldered to the suspected traces, and watch the obvious visible sparks.  Less ideal than a regulated DCKV supply is one of those BD-10 hand-held Tesla coils.  

Even try this: figure out how to detect any sparking at all (such as a de-tuned AM radio nearby.)  Apply your kilovolts to the pcb to create sparking at a mysterious location.  Now, for each suspected sparking location on the board, cover it with a blob of silicone caulk.  Assemble everything and apply the kilovolts again.  If the same sparking is still there, then do it again, another blob of caulk at another spot. Eventually you'll hit the offending location.

I've seen pcbs with proper HV-holdoff spacing around certain traces, but where nobody checked for HV standoff going vertically through the board.  At one spot the 2KV PMT supply trace went right over a piece of internal groundplane.  Multilayer board, so very thin epoxy-glass between those layers.  After a few years of operation, the HV chewed right through the pcb, creating an unwanted carbon resistor.  See Cary Spectrometer PMT shorting.