Detecting very high voltages
I work for a plastics molding company and we manufacture containers that we need to verify do not have holes in them. I am looking design a fixture that will apply a very high voltage (about 15-30KV) to the surface of the product to see if the electricity finds a path through the material. I then want to sound an alarm if a path is detected.

That sounds like condom manufacturing!

I'm struggling to find a way to detect the current flow without blowing out (due to the high voltage) the electronics.

Large resistors: &quot;blowing out&quot; happens because high current flows. The relationship between current and voltage is I = U/R, so pick a large R.
In fact, pick two R, one large, one small, and build a voltage divider.
Driving a commercially available octocoupler (i.e. not the split thing that you described, but an LED and a photodiode in the same black package) with the output of your voltage divider might still be a good idea to protect your system if something should go wrong with the high voltage.