Found this by googling "how to bench test Porsche 911 speed and reference sensors" First results were from Pelican. First suggestion was a thread asking about Reference Sensor Testing.
"Unplug both black plugs and measure around 1000 ohms resistance between the bottom two of the 3 terminals of each black plug, on the engine side of course."
He's referring to the plugs on the left side of the engine bay the top and middle I think are speed and reference. The bottom plug should be your cylinder head temp sensor IIRC
Here's the thread
http://forums.pelicanparts.com/porsche-911-technical-forum/893183-reference-sensor-testing.html