Drawing By Keithonearth (Own work) [CC BY-SA 3.0], via Wikimedia Commons
Derailleur-equipped bikes almost always have dished rear wheels. This means that the hub flange on the right-hand side is offset toward the center of the hub in order to make room for the sprockets. A dished wheel is weaker against lateral forces.
A dishing gauge can be laid over a wheel to measure the offset from the rim to the place where the axle contacts the dropout. Then, the dishing guage is moved to the other side of the wheel. The distance should be the same.