There are two different wiring methods in use in the US: Conduit and cable.
Conduit is a metal or plastic enclosure holding individual wires (OK, it could have cable, but then just treat it as cable for this discussion). If you use conduit, then the two hot wires of a 240V or 120V/240V (like an MWBC) can be the same color. In fact, using the same color can save a bit because you can just use three colors (black for hot, white for neutral, green for ground (and no ground with the right type of metal conduit)) for just about everything. So if you have conduit and the wires are the same color then you need to actually figure out which wire is which.
Cable always comes the same way: 2-wire = Black & White. 3-wire = Black & Red & White. For an MWBC, of course, you would use 3-wire cable. With a 3-wire cable, there is no ambiguity. Properly wired, black and red will continue through the system until you have a split, at which point you can switch to 2-wire cable.
Open up your receptacle boxes. If you see black & red & white, then unless there is some really poor wiring (i.e., black & red swap at some point), they should match all the way back to the breaker. If you see just black & white then you trace back until you find where this split off from the MWBC to figure out whether the black goes to the black wire or the red wire back to the breaker.
The only problem is if you have conduit and the two hot wires to the breaker are the same color. In that case, shut off the double-breaker and remove/cap one of the hot wires and then turn the breaker back on, and you should be able to figure out easily enough which receptacles are connected to each hot wire.