The easiest way to measure backspace is to lay the wheel face down onto the ground so the backside of the wheel is facing up.
Take a straight edge and lay it diagonally across the inboard flange of the wheel. Take a tape measure and measure the distance from where the straight edge contacts the inboard flange to the hub mounting pad of the wheel. This measurement is the backspace.
The photo below shows three wheels with 2", 3", & 4" backspace.