The cleanest way is to put a on the led , with the + lead on one end of the led and the - lead on GND. You're looking for the forward voltage of the led so if you put the meter on the cathode you won't get much but on the anode you should get 1.3V to 2V. Now look at the traces going to the led and find the current limiting resistor for the led. If it is connected to the cathode then the other end of that resistor would go to GND. In that case the voltage across the led should be close to +5V and you can solder a wire to the Anode of the led and bring that out to the arduino and connect that to a digital input and do a digitalRead.
If (ledval==HIGH)
{d13= digitalWrite(OUTPUT,HIGH);}
If the current limiting resistor is connected to the Anode of the led on the breakout , then the voltage you want to tap is on the other end of that resistor (about +5V) and that's where you connect your wire. The breakout already has GND connected so you only need the one wire. Either way , you are looking for the signal that drives that led, to read with your digitalRead and turn on
whatever based on that condition. The longer way is find the analog threshold that is set by the trim pot on the breakout and that's more complicated.