How to tell whether a point is to the right or left side of a line

You look at the sign of the determinant of

| x2-x1  x3-x1 |
| y2-y1  y3-y1 |

It will be positive for points on one side, and negative on the other (and zero for points on the line itself).

Use the sign of the determinant of vectors (AB,AM), where M(X,Y) is the query point:

position = sign((Bx - Ax) * (Y - Ay) - (By - Ay) * (X - Ax))

It is 0 on the line, and +1 on one side, -1 on the other side.

Try this code which makes 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.