Checking status of a wire

Hi, I have an external circuit with an LED. I want to use my arduino to perform a task when the LED turns on. How can I check if the wire is live or not when it's a separate circuit from the arduino?

Thanks!

Either use a phototransistor or LDR next to the LED (ie, a light sensor), or tie the grounds of the two circuits together, and use normal techniques.

Which is appropriate depends on the circuit you're trying to interface with (mostly on how much you know about how it works)

Perhaps I should be more specific.

I have a water softener (http://www.ecodynewatertreatment.com/softeners/view/whes44/), and would like to receive an email whenever the salt level is low. The softener is digital, but has an LED that I believe I can solder into (+ and -) to test the wire.

I don’t know that a phototransistor is my best bet. I’d prefer to get a read on the wire having current or not if possible.

See if you can find a ground location on softener control board somewhere, measure from that across the LED when it’s off & on and see what kind of readings you get. If you can see <1.5V and >3V on the LED anode, then you can read it directly into an Ardiuno digital pin, connecting the Arduino Gnd to the softener control ground.
Otherwise, leave the grounds separate and connect the Arduino Gnd to the LED cathode (usually a flat spot on a thru hole package, otherwise use your meter to find the low & high pins) and a digital or analog pin as needed to the anode. The Arduino is then separated from the softener just like the meter was.

Once removed a LED and put an optocoupler in the place. It workes great but you do need to know the voltage and current of the LED to select the right optocoupler.