collision detection - Given a plane and a point, how can I determine which side of the plane the point is on?

Given the point

Vector pos = new Vector(0.0, 0.20156815648078918, -78.30000305175781, 1.0);

and the plane (triangle)

Vector a = new Vector(-6.599999904632568, 0.0, -78.5, 1.0);
Vector b = new Vector(6.599999904632568, 0.0, -78.5, 1.0);
Vector c = new Vector(6.599999904632568, 4.400000095367432, -78.5, 1.0);

I want to get a plane normal pointing in the direction of pos

//Getting plane normal
Vector ac = Vector.Subtract(a,c);
Vector bc = Vector.Subtract(b,c);
Vector planeNormal = Vector.CrossProduct(bc, ac);


//Testing which side of the plane the point is on
double dprod = Vector.DotProduct(planeNormal, pos);
if (dprod < 0)
{
    planeNormal.Negate();
}

But this method is wrong. The resulting planeNormal points in the negative Z direction, so it should not be negated. Is there a best practise for this? Please help me, I fail massively @ math :)

Answer

Your method is mostly correct but misses one step. You can't simply use the point's position as the vector to get a dot product with, you need to create a direction vector from a point on the plane. Any point on the plane will do (the direction doesn't need to be exact) so just use one of the corners.