All wheels have the size, width and offset cast or stamped into the back side..usually on a spoke like the pics show. The offset is usually metric which can be converted to the other form using many of the tools found on the web.
In the example below, which by the way is the rear of a ZForce, the wheel is a 14X8.0"..or we just care that it's 8"s wide:
And that the metric offset is +46.5 which is the amount from the center of that 8"s that the mounting surface is off set to the outside which increases the amount that goes inward:
Using a converter, you enter the width of the wheel, then the metric number. The converter then gives you a number in inches. That number is the inset or the first number of the common offset. In this case it is 6.33". So, 8"s minus that number will give you the other number you are looking for, the outset. From this we see that this wheel is a 6.33+1.67.
You will never find an aftermarket wheel with this offset but you will find something close like..6+2..or a little more outward and less inward like 5+3...or an even split 4+4.
Just take one front and one rear off and get those numbers off the wheel. Google "Offset Converters" and get your answers.
This is one site:
http://www.gtsparkplugs.com/Offset-to-Backspace.htmlGood luck.