Went to replace a crappy looking socket in a 1909 house (painted the room), and found what I now know as BX armored wire. However, yanking out the old socket, I find there is two black wires and one white (with a red stripe) wire. It's hard to tell, but in tracing the cable back to the junction box all three wires are connected to similar BX. However, it looks like the white might be being utilized as a ground wire, as it's the only connection not capped/taped...just left naked.

Everything I read about BX is that it was just two wire (B & W), with the armor acting as the ground. Here, I have to assume one black is hot (which one I don't know), but if the armor is the ground...what's the white doing?

So, is there a procedure or tester that can help me identify hot, neutral and ground?