You could use an LDR (light dependant resistor) to check whether the LED is on. Use black tape or other stuff to make sure only the light from the LED reaches the LDR. Connect the LDR between an input pin of the PIC and ground, and a say 1 MOhm resistor between the same input pin and +5V (pull up). As soon as the LED lights the resistance of the LDR will go down, pulling the pin low. You might have to experiment a bit to get the right value for the resistor.