How to identify Fuchs size?
 

Can one measure the width of a wheel without removing the tire?

Are there any markings on a Fuchs which determines or denotes its size?

Without removing the tire? Yup, pull the wheel off and flip it over.....there is a serial number, a "fox" logo and the size. If not all there, it's a fake.

The serial # on 911 offset wheels use 911.XXX, 944s, 944.xxx, etc...

With the wheels still on the car, I just attempted a measurement by using paint stir sticks flat against the rim and measure in between. Found the following:
front 6 1/2 inch od
rear 7 3/8 inch od

so I figure I have the stock 6 & 7 in 16's that I thought I had.
Sound about right?

Just look at them. 6 are perfectly straight from the petal to the rim. 7's have 1 inch of lip angled. 8's 2 inchs of lip. 9's 3 inchs. The wheels are just about identical except for this addition spacing, (same backspacing offset).