Finding fabric and thread thickness
While a caliper would be a great tool for calculating something like this most people do not have access to one. Coupled with the fact that the level of precision it provides is lost in most crafting endevours. There are some simpler techniques that you can use to get similar results.
For Fabric
Ever tried to measure the thickness of paper? Perhaps in school. Its near impossible as most rulers don't match that level of precision. What if you stacked multiple pages together? Putting 10 (or 20 or whatever) sheets together and you can easily measure the total and divide by quantity used.
For fabric reems this can get harder but you should still be able to fold it over itself several times without cutting it or anything. You could even roll it up and measure the outer layers. Either technique you use get you a decent figure to work with.
For Thread
When it comes to thread thickness I would like to borrow from another answer in this stack. Wrapping the thread around a ruler enough times to get a nice round number like a mm,cm or inch makes the math easier. Can you fit 10 rounds of thread in a mm? Then your thread is 1/10ths of a mm. Keep wrapping until the math is simpler.