Well I think of phase differences as the fraction of one oscillation completed by a point on the wave. So for example, if in a point on a sin wave is at +1 or -1, the point would have completed 1/4 and 3/4 of the full oscillation respectively. Then, you multiply this fraction by 2pi as 2pi represents one full oscillation. This gives you the phase difference between the starting point and the point on the wave. To find the phase difference between two waves, imagine the first point as the starting point and find the phase difference of the second wave in respect to this. It's basically a matter of perception.