The typical case is that a 3 conductor (plus ground) cable is used for a multiwire branch circuit, with two ungrounded (hot) conductors on opposite legs of the supply supplied from adjacent full size circuit single pole breakers and a common neutral. Those two adjacent single pole breakers are required to have a handle tie.
So if you are qualified to do so, you can kill power to the panel, remove the dead front, and inspect the wiring. Typically 3 conductor cables have a red wire, and you can look for any red wires that terminate on a single pole breaker. Trace the red wire back to its entry to the panel, find the associated black wire, and trace it back to a breaker, which should be adjacent to the breaker with the red wire.
Cheers, Wayne