Sensing current on an LED

Just be careful not to accidentally drive 5V into that signal - you could add a 10k resistor in series to prevent that possibility.

[ oh, and one caveat, its possible that the bluetooth module doesn't drive that signal high enough (due to the current load from the LED) - it needs to be 3.0V or more to guarantee working with the Arduino input - you can measure this voltage when the LED is on to double check this ]