Hi,
I suppose you could use anything that could read pretty accurately at that size.
It's probably thin enough that you'd have to use a ruler with very fine graduations.
I suspect it could be in the area of about 0.5mm-1.0mm.
Any vernier caliper could do it, or a micrometer, or a very finely graduated engineer's ruler. The more accurate the measurement, the better. But anything close is better than nothing.
If you have the time and inclination to try it, I'd appreciate the help for the cause!