Down in this thread you can find the instructions. Basically you want to measure between pin 87 of the low speed relay socket and pin 87 of the corresponding high speed relay. The ballast resistor is connected across these pins.
The claim is the resistor should be 0.55 ohm. I have a standard dmm which cannot measure this accurately, my leads have nearly half-ohm by themselves. I think you simply want to make sure that you measure 1 ohm or less meaning the resistors are intact. They should only fail by presenting an open circuit, not a short,
afaik.
Radiator fan not turning on when AC is off - 986 Series (Boxster, Boxster S) - RennTech.org Forums