First, if you put more than 5V into the Arduino, you can damage it.
Do you have any idea what kind of voltage you'tre getting out of the pot. If the "hot" terminal on the pot is powered to 24V, that's a problem.
You can use a voltage divider (2 resistors, something like a non-adjustable pot) to knock-down the voltage, but the total resistance needs to be about 10 times the resistance of the pot or the voltage divider will affect the voltage coming out of the pot.
OK... You just need a common ground (connect the two grounds togeher) and you can read voltage from a separate source.
A voltage divider would work as long as it won't affect what the arduino is reading.
Read voltage from a separate source?