How do you determine which side of an open vector is the right or left?

Select the vector and press N, the short-cut key for Node Editing. The small green square at one end of the vector is called the Start Point. If you start at the Start Point and follow the vector toward the node at the other end, the right side is on the right, the left side on the left. It may also help to visualize walking along the vector from the green Start Point to the other end. 

eg. Right hand side.

vector-side