You need to establish one phase as your reference then see which of the other phases is ahead of the other. The easiest way to do this is to use zero crossong detectors and simply see which phase crosses zero first after the reference.
Alternatively, you can use three ADC channels and note whether the samples on each is increasing or decreasing to derive the exact phase difference. If it's just to check them rather than measure them, the first method is simplest.
Brian.