What are some ways to test the r-values of different cob mixtures?

I have a small rocket stove project in mind that I'm trying to make without buying anything, so, using only components in my household.  Our soil has a good percentage of clay, so I'm gonna be using cob, but I want a way to test what mixtures and additives have the best insulation properties of the materials I have access to.   Shredded paper?  Cut grass?  Husky fur, because our dog is shedding this month?  Something else I haven't thought of yet?   I don't know what would be best, and I would have fun testing it.

Hi and welcome to Permies!
In any cob mix you want clay, sand and fiber. So a good first step would be to make some test bricks from the soil you have. Just make it into a workable moist mass and form some "pucks". After drying see how much they crack and how much resistance they have to compression.

Could you elaborate more on the project you have in mind? A lot of the answers are "it depends", so it really depends on what your plans are.

Insulation in heated cob (hot enough to burn out the organic filler) comes from well distributed small pores. Straw would be one of the best for this, as where it does not get hot enough to char, it is structural, and where it chars, it leaves hollow tubes of space which give insulation. The more straw, the more insulation, as long as the fired cob still holds together. Dried grass leaves flat thin voids, and would be less effective as insulation. It does make nice workable cob for finished surfaces, as the fine grass allows quite a smooth surface.

Sawdust is another common filler for insulating cob. It would give little strength, but in higher proportions could make a well insulating material.

Testing for numerical insulating value is likely beyond the capacity of any home workshop; I would make up panels say a foot square by a few inches thick (all identically sized), and put them equally spaced around a uniform heat source, and see how long each sample takes to feel warm or hot. This will give you a useful comparison between your available materials.