I have prepared a leaf-rich garden patch (see here). To promote decomposition of the leaves I have planted a lot of beans and peas to promote nitrogen fixation in the soil. However, I don't know if there are any Rhizobia bacteria in the soil. How do I figure this out? How do I figure out their concentration?
I don't know how to figure out their concentration, but figuring out if you have any is easy if you're willing to be patient.
Keep in mind that there are different species of Rhizobia that work with different groups of legumes (e.g. clover needs different bacteria than string beans).
At the end of the growing season, carefully dig up the roots of your beans and peas. If they have nodules (see picture in upper-left) then you have Rhizobia. I'd assume that heavier nodulation means a higher concentration of bacteria.
As far as I can tell, testing concentrations are not something you can easily do at home. If you want to ensure that you have Rhizobia on a home-garden scale, it will probably be cheaper and easier to just buy a packet of inoculant and coat the seeds prior to planting. In your case, this isn't possible because you've already planted the seeds.
For what it's worth in terms of breaking down your leaves, I'm not sure you need to worry much about nitrogen. If you have any worms at all in your garden, you'll find tons of worms in the leaves now. Work that soil gently (if at all); avoid chemical fertilizers, pesticides, and herbicides; and they'll multiply like crazy and munch it all down for you.