To determine which side of the line from A = (x1,y1) to B = (x2,y2) a point P = (x,y) falls on you need to compute the value:
d=(x-x1)(y2-y1) - (y-y1)(x2-x1)
If d < 0, then the point lies on one side of the line, and if d > 0, then it lies on the other side. If d=0, then the point lies exactly on the line
To see whether points on the left side of the line are those with positive or negative values compute the value for d for a point you know is to the left of the line, such as (x1-1,y1) and then compare the sign with the point you are interested in.