Measure or detect high current pulse 600A+/15V without osciloscope
If the cable is 50mm2, that's 0.35 mOhm/m.

If you use, say a 10cm length of cable as a shunt that's 35µOhm, with 500A current you get 17mV voltage drop. You'll need a low offset opamp to amplify this, say with a gain of 286 so it displays 1V/100A. You can also use a Hall current transducer or a clamp-on ammeter with a signal output.

Then a peak detector and a multimeter.

If you care more about price than accuracy, even an old LM358 should do the job, although its 2mV max input offset will also be amplified and displayed by the multimeter... you'll have to remember to substract it from the measurement.

This should be easy to build with parts from the "junk pile" and a 9V battery.

You can check it works by using a voltage divider on a battery to make a known voltage, say 10mV for example.