http://stackoverflow.com/questions/1560492/how-to-tell-whether-a-point-is-to-the-right-or-left-side-of-a-line

 use of a cross product:

public bool isLeft(Point a, Point b, Point c){
     return ((b.x - a.x)*(c.y - a.y) - (b.y - a.y)*(c.x - a.x)) > 0;
}

Where a = line point 1; b = line point 2; c = point to check against.

If the formula is equal to 0, the points are colinear.

If the line is horizontal, then this returns true if the point is above the line.