Besides the obviose of looking on the inside of the wheel for the stamping how would you measure the width?
Is it from lip to lip or bead to bead?
Was looking on my tires for tire pressure and read "tire must be mounted on 10" rim" so what I thought were 8.5 wide wheels I think are 9.5 or 10 depending on the measuring point.
Anybody know?

 

Bead to bead exactly where the tire bead meets the rim. The total lip to lip measurement can vary a lot between different wheel manufacturers and rim designs even given the exact same width spec. However, the bead to bead measurement will always be exact.

 

since tires are of standardized bead sizes and HAVE to fit the wheel

 

Yep. Exactly