Megger, generic term for insulation resistance, put 500/1000V down the windings to check for resistance, or lack of it to earth, but you probably haven’t got one of those then by the sounds of it……
If the motor runs its unlikely to be that to be fair, could be that the motor is in fact shot, or you haven’t setup the inverter correctly, pretty hard to tell from behind a keyboard to be fair.
NOTE: Disconnect motor from inverter before doing this !!!