i bought two single wheels yesterday for 30 bucks these are brand new that were display models. well i know they are the right bolt pattern but no idea on backspacing and offset. they will be used on the rear only as drag wheels. how do i check backspacing and offset?

crappy pic:

 

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 padof the wheel.

Cut and paste from here:
https://www.rsracing.com/tech-wheel.html