I am trying to add a secondary box to add some features to a project. Basically I have a stand alone 4 LED driver that opens and closes the negative leg of the LED to turn it on or off. I need to be able on a second arduino completely independent to measure that opening or closing and then do something else based on that.

Is measuring current across those legs the easiest way? Im looking for probably 20-50ma of current. Is there an easier way? Can I just put an analog pin and hook it up to the resistor (180Ohm or so) and just get a reading from that? It doesnt need to be anything accurate as far as a definitive value. Just needs to know if the LED should be on or off.