There is no safe way without tracing it back. If you can get to the ends maybe an non contact tester ( idiot stick ) will tell you, but in order to do so you probably have to handle the wire.Safest way without tracing it back.
That's about all you can do. Usually it would be impractical to shut the main off, on big jobs that i'm usually on.It's hot until proven otherwise.
I would crack it open and stuff a non contact tester in between the metal.
Right...and I think it's a very good question for some one just starting out to ask cause like I said I almost bit it in my 1st yr with this same thing. Really no one should ever leave an mc like that but...Deano...use insulated channel locks or something to break the steel or aluminum so you can get your touch stick to reach the wires inside.But you can strip the bx/mc pretty easy.