I found an oscillator that has the X-Y function where the left channel is represented by the Y axis and the right by the X axis.
If a mono sound is played through the 'scope a line appears from bottom-left to top-right and is of course in phase. When a stereo signal (eg the track I am writing) is passed though the line changes into a "cloud" but the direction of the cloud was clearly bottom left --> top right indicating that the signal is in phase. If the line changes direction by 180 degrees then the signal is considered out of phase.
Hope this answers your question!
You can read the article I found that brought this to my attention at...
http://www.creativesynth.com/feature...cretBox_1.html
Martyn