How do we do this? Other than eyeing it and using the common metal gauges?

Most wire will have the size imprinted on it. Other methods are an experienced electrician can usually eyeball it and then you can always compare the size of the wire with the protective device feeding it and see if that makes sense.

I used to have a measuring device I got from a cable manufacturer. It was a flexible plastic strip that you wrapped around the cable to measure the circumference. It was marked to directly measure wire guage or MCM for standard insulation thicknesses.

Don't know what I did with it. Wish I still had it. Anyone know where I can get another one?

That would be sweet!!! All to often the size has warn off or is written in an inaccessible area of the cable.