Yes the insulation resistance should be measured with the insulation tester...
I said earlier that it is unusual that if the motor would be bad, that the fuse would blow. But now that i see what VFD you have, i wouldn't be surprised if it's protection functions are that bad, that it would rather blow the fuse than activate the short circuit protection
No offense, but those chinese VFDs are quite crappy...
Best option would be to get a 3ph motor somewhere for the test, maybe a neighbor or friend has one. Any small 3ph motor with the 230/400V rating will do it, just set the U/F first, accordingly to the new motor. Probably 230V/50Hz. And you will need to wire it in delta if it was previously connected directly to 400V mains.
But i wouldn't be surprised that both are toasted, the motor and the VFD. If the motor has a short to the ground and VFD's protection functions are not working properly, there is a high possibility that IGBTs are also gone. Did you try checking the IGBTs, as i mentioned before? DC bus to output? Should see the diodes of the IGBTs...