It is easy to get the angle between two vectors using Vector3.Angle, but this only gives an absolute value for the angle. How do you tell if one direction is to the left or to the right of the other? It seems as though it should be easy, but in fact it's a little bit subtle. I've attached a function (in both JS and C# versions) that illustrates how to go about it. You need to supply a forward direction, the direction that you want to check for left/right of forward and an up direction. The functions return -1 when the target direction is left, +1 when it is right and 0 if the direction is straight ahead or behind. As is so often the case, the magic ingredient in the function is the cross product.