Current flowing through a wire creates around itself a magnetic field. With the manner in which a three phase motor's stator is wound, and the manner in which a three phase generator provides power to the distribution system, the net magnetic North created within the windings will rotate around the stator. The direction of rotation is a function only of the phase sequence of the currents flowing through the stator. Once the magnetic field is rotatating in one direction or the other (i.e., CW or CCW), the rotor will be driven in the same direction.
There are only two possible sequences (ABC or ACB), regardless of how you made the connections. All other sets of letters are identical on one of these two sequences. Thus, there are indeed only two possibilities, and the odds are 50/50.
That said, is the wiring choice really arbitrary? Are there no clues from the color codes sometimes used in wiring systems, or in the information provided by the manufacturer? Never having connected a motor myself, I would not know whether the terminals are labeled in any useful way.