Both of the main federal land management agencies as well as NRCS have manuals that describe how to collect data for vegetation and soil health. However, it is quite labor/time intensive and there is often little to no budget allocated for the collection of the data.
IMO/E unless an area is very, very highly eroded and/or has a very bad infestation of non-native species that are drastically different than what occured there naturally (ie perennials vs annuals) soil health is most often not an issue. These areas are fewer and farther between than most folks would think. The soils change and degrade much, much slower than the vegetation. Not saying that many areas on public land the soil health can't be improved or that productivity has not been decreased. Take a look at the State and Transition Model of an Ecological Site for an area you are interested in. These are used to describe the different types of vegetation that occur at various "states" of soil health as well as the restoration activities to improve them and the causes to transition them to a lower state. If the plant species growing in an area are those of the reference community, even if their relative proportions are off, I highly doubt the soils have changed to the point of drastically changing the nutritive quality of the forage.