Just test the impedance from the grounding point to the negative lead on the main starting battery.
Then test the impedance across the factory grounding strap on the factory battery.
If it's higher than the original grounding strap, i'd run at least a 4 gauge wire back to the main battery, IN ADDITION to the 0 gauge strap to the car.
It's gonna depend on the car, and how much time you spend preparing the grounding spot, or finding a suitable bolt to ground it to.