You can use a pair of resistors as a voltage divider to bring the 12V - ish signal down to the 0..5V range that the Arduino can handle. I suggest you provide some form of protection against overvoltage, because the incoming voltage could easily exceed 12V.
After you have read the analog value, the map() function provides a convenient way to transform this to a voltage - it would be reasonable to represent the voltage as a number of millivolts.