I cannot imagine a wiring configuration with only two phases that would tell a phase rotation for all three. Could you share a product manufacturer and model?
This one has three wires connected. http://www.traknet.com/wei/aemc9.html
My understanding is many rotation testers are really small induction motors that rotate in a direction relative to the phase rotation measured/applied.
Maybe an electronic meter could see phases AB, determine that they are 120 deg out of phase and assume that the third phase is either before the A or after the B, resulting in clockwise rotation either way ABC or CAB.
[This message has been edited by Ron (edited 08-13-2004).]