The amount of positive or negative offset is determined by the difference between the wheel center and the hub face of the axle. On a loose wheel this can easily be determined by laying the wheel down flat on a flat surface and measuring through the center hole from the wheel face to the surface that the wheel is laying on. After taking this measurement, flip the wheel over with the other side laying down flat and repeat the measurement. The difference in these two measurements, divided by 2, will give you the amount of offset.