[Keri] selects them not with his golden ears, but by wiring both pots together as a Wheatstone bridge. A meter between the two wipers would detect any current due to a mismatch. A Wheatstone ...

half-bridge or full-bridge Wheatstone bridge circuits. Pictured here are tension bars with strain gauges built into them for measuring strain. The one with only one strain gauge is a quarter-bridge.