There is an ID stamp on the rim, on the backside.
The wheels I have here are 'JJ' 15"x7", very common.
If you measure between the tire bead surfaces, they are 7" wide.
But if you measure them from edge-to-edge, they are 8" wide.
Everybody refers to these as having 4-3/8"' backspace.
Mounting surface is 4-3/8" from the edge of the rim on the back side, and 3-5/8" on the front side. .....In other words, there is a 3/4" offset.
If you have wider wheels made, they should maintain that 3/4" offset for them to fit the same.
However, my '66's will have better tire fit if offset is increased 1/2" to 1-1/4". That centers the tires in the wheelwell.