Here's another clever solution, submitted by an inSync reader, for determining the polarity of unlabeled long wire runs (see TTOTD from 12/15/99 for original question).

"Here's another way to check polarity, useful when making a loop is difficult (and doesn't require taking the wall box apart to look at the wires, which really isn't too bad usually). First, set your multi-meter to Ohms and on the diode test (usually 2k or 200 ohm) scale. Measure a diode, noting which way the meter deflects as the leads are connected. Try both ways; the meter will probably do nothing with the diode hooked up one way (e.g., (+) to cathode and (-) to anode -- the cathode usually has a line around the body of the diode). Now place the diode across the two wires at one end (in series with the wires) and measure the other end (in the next room, building, wherever). This will allow you to determine which wire is connected to each side of the diode on the other end. No extra wires needed!"