ADC output of 2 sine wave with equal amplitudes and different phases
If you only have the two sine wave inputs and no other reference there is no way to distinguish one from the other if one is 180° offset from the other. 

On the other hand, if the phase difference is (say) 90° as in two-phase (quadrature) signals, you can tell the difference by simply looking at the other voltage when one crosses zero in a given direction.