Using a UA741 IC as sensor for an existing AA LED
simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. Eavesdropping with an opto-isolator.
Inserting an opto-isolator into the original LED circuit provides the best means of doing this as it provides complete electrical isolation between the two devices. It does require minor modification of the source but it should be a trivial exercise. The opto-LED will drop about 1.4 V but it seems that you have plenty of voltage to spare.
D3 can, of course, be connected as shown, above R2 or below D2. If you can lift a leg of R2 that would be the easiest.