They will say the backspacing in the description if you order them online. If you are looking in person, set the back side of the wheel down on a flat surface. Measure from the flat surface to the wheel mounting surface. That would be the backspacing.
92 YJ, 4.0, D44/60, 5" BDS springs rear, SOA front, 1" booms, 1" BL, 35x12.50x15 MTZ's, 4.10 gears. It's never done though.