you can have a phase sequence indicating meter. So, if the phase sequence is ABC, pointer of the meter rotates in clock wise direction. With sequence of ACB, pointer shall rotate in anticlock wise direction.
If you want to believe the generator manufacturer, normally, looking from the non driving end of the alternator, when shaft of the engine is rotating in anticlock wise direction, phase sequence would be ABC. at its terminal.
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Exploring the Science of Electricity
The only sure way to tell is for your to grab the generator shaft when it's operating at full capacity. If your body is flung across to the north side of the room, the phase is ABC. If it's flung to the south side, it's ACB.
If you hit the east or west wall it's CBA and BAC respectively. Either way, we're all winners. Grab on.