To measure backspacing lay the wheel on a surface face down. Place a Straight edge across the top of the wheel, and then measure from the mounting face to the bottom of the straightedge and there you go.
I found this image that may help explain things better.
Just in case you need to know how to measure offset it is a little harder to understand, to get the offset of a wheel first find the centerline, and the offset is the distance from the mounting face to the centerline of the wheel. It's almost always expressed in milimeters, and either positive or negative. If the wheel has a positive offset that means that the mounting face is in front of the centerline relative to the face of the wheel, If the wheel has a negative offset that means the mounting face is behind the centerline relative to the face of the wheel. I'll try to find a diagram online
Also if you need to convert offset to back spacing simply divide the wheel width in half, and add the offset whether it be negative or postitive. Ex say the offset is -35mm on an 8" wide wheel.
8"/2 = 4 + the offset (-35mm)
= 4 + (-35mm) and [35mm = 1.379"]
=4" - 1.379"
= 3.621" backspacing