[old_tv_nut]

To check for signal to ground short in a cable, disconnect the cable from the equipment and then connect one ohmmeter lead to the signal (center conductor) and the other ohmmeter lead to the ground (outer conductor/shield).

Regarding grounding, your turntable should have its own ground wire to connect to the ground terminal on the amplifier.

I don't know what to tell you about your "jury rigged" cartridge wire, but I think if you have the correct wire the cartridge and cartridge shell should float and only be grounded via the connector at the amplifier. It's been a looong time since I wired up my turtable, but I think there is no way provided to connect the cartridge wire to the turntable chassis.
I hope somebody else chimes in here.