Evaluating the soils in your vineyard requires, generally, several steps
The first step is to research the geologic parent material of your soils, which can be found in the Natural Conservation Service and Geologic Survey source: The Soil Survey. This will give you broad, baseline information about your soils and is a good place to start when assessing your site for farming potential. UC Davis soil resource lab has more information regarding the soil survey and how to navigate it.
After getting a baseline using the soil survey it is a good idea sample from multiple locations on your property, with a backhoe or a shovel. If you have never, submitting an aggregate sample to a soil analysis lab can get you information such as: Physical properties and texture, water holding capacity, pH, cation exchange capacity, macro and micro nutrient values, organic matter content, and other parameters if desired.
In the foothills, checking phosoprus could be important because it may be tied up or "fixed." Another component important to the foothills is considering a lime applicaiton suggestions for soils that may have a pH lower than 6.5. Most foothill soils are indeed acidic (pH less than 6.5), however many growers do not add lime nor try to adjust the soil pH. Instead, they consider the acidic soil pH part of foothill "terroir," which is a term to describe environmental factors that affect a crop.
Check our List of Laboratories for Tissue/Soil/Water - Agricultural Analysis to determine where to send your sample.
Soil depths can vary on a single site.
A single vineyard site can vary greatly with respect to soil depths. The difference in depth at a single location may varying by feet, and these differences are important to know for proper management of vineyard water and nutrients. Depth can affect the water holding capacity, the vigor capacit of the vines themselves, and the ability for nutrients to leach. Often factors such as soil depth become important terriors of the foothill grapes.