Vintage stereo hi-fi amplifiers commonly use dual-ganged pots for volume, balance and tone controls. A mismatch of resistances (and resistance ratios) between channels may be audible, throwing the amp off balance.
This method uses a Wheatstone bridge formed by top (wiper-to-positive) and bottom (wiper-to-negative) parts of potentiometers, which ideally are of the same resistance for the left and right channel. In this case, the wipers would be equipotential with no current flowing between them. In real life though, resistances will be slightly different, causing a potential difference (a.k.a. voltage) to develop between wipers, which will make a current flow through the meter
Testing is as simple as it gets, all it takes is turning the pot and observing the meter's indication along the way. You can even draw a mismatch vs angle curve!
The best matching can still be obtained by using stepped attenuators with closely matched parts between channels.