I.e. n0 for A and n1 for B. Plane divided by plane CD.
First, find the center:
p0 = (C + D) * 0.5
Now, take the relative vector from the center to the point in question:
p1 = (X - p0)
Finally, take the dot product with the normals you might want:
d0 = n0.dot(p1) d1 = n1.dot(p1) ... etc.
Then, check to see if its greater than zero
abovePlane0 = d0 > 0 abovePlane1 = d1 > 0