You will have either 3 or 4 wires supplying the control box on the tub. The bare copper wire will be ground. The other 3 should be (I say should be because I have seen some weird things over the years) black, red and maybe white. Black should show 105-125 volts to ground and red should show 105-125 volts to ground. White is common and is required to run any 110V compononts in your tub. It may or may not have this. Turn power off and check incoming to the GFCI breaker first. Then power back up and check incoming to the tub.