Grumpy_Mike:
Wire the common +ve to the 12V supply and measure the voltage between the ground and the wire you want to connect to the arduino. You will see the voltmeter will read 12V. You don't want to connect 12V to an input as I said.
Alright, so I've got the +ve from the 12V supply going to the VIN and the LED Strip. The ground from the supply going to the Arduino ground and LED Strip ground. And the LED strip control going to the A0 Output. The voltage drop across the A0 output and ground is <1V.