With the wheels on someones car what's the easiest way to tell if they are 8.5 / 9.5 or 9.5/ 10.5?

 

A ruler! /Thread

 

^its hard even with a ruler.

you really cant without taking the wheel off the car.

 

Are you talking about width or offset. The numbers you're asking about seem like you're looking for width. If you're looking for width, take it to a tire shop. They have large plastic calipers that are used to measure width for wheel balancing calibration. Also, most quality wheels will have a stamp or sticker somewhere.