It might be a good idea to put a 10k ohm 5 watt resistor in series with an LED on both ends of the battery pack (144V for exampl) to vehicle ground. If either LED lights brightly there is significant leakage. Otherwise this should give you a voltage of about +/- 72 volts from either end to chassis. The 10k resistor will limit the current to about 14 mA maximum if you should touch any part of the battery pack while you are grounded elsewhere. This is enough to give you a bit of a shock but not enough to cause serious harm (unless you jump and fall or hit something).
There is probably enough leakage and capacitance in a complete system including controller and motor to give you a shock. And insulation breakdown of wiring or conductive dust from the brushes and commutator may provide an even stronger current. So you could even replace the LEDs with SSRs and trigger an alarm or trip the safety disconnect if either one sees enough current to turn on.