There are two ways to approach this problem. The first is, there's a tester you can purchase cheaply at most hardware stores that can be placed close to any one of the two wires without contact. The hot wire will activate the tester electromagnetically. The other way is to tie or touch an insulated wire (#14 or #12 thhn/thwn) to a known ground (such as a galvanized or copper cold water pipe under a sink). Place or clip one lead, of your high voltage tester, to the other end of the wire that's tied to the known ground. Then place the testers other lead on any one of the two wires feeding the sconce, once again the tester will signal when touching the hot wire, If the tester does not signal a voltage, either the supposed ground is broken and incomplete, or the circuit feeding the sconce may be shorted. The first way mentioned is safer and easier. Remember to read your tester instructions carefully. Also in many old houses knob & tube switch circuits were wired on the neutral leg, so flipping off the light switch may not de-energize the circuit, so be very careful!!!! If you have any doubts please call an electrician!!